I'd like to add on as well as give my opinion regarding a few of the things that have been mentioned in this thread.
Obviously you'll want a plan of attack before you leave enrollment in a professional school to embark on the potential of entering another professional school. Getting into the PsyD was a feat in itself. Though a lot can be said for professional satisfaction and doing what you love, other aspects that often don't get as much consideration are job stability and security. Often those with both security and being able to do what they love will tell you to, above all else, do what you love, which is true but they may often neglect security since it's something they already have. What I'm getting at is that on one hand you don't want to walk away from your PsyD and for whatever reason not get into medical school and on the other hand you wouldn't want to get through the PsyD and end up struggling in clinical practice wishing you had a bit more stability in your career (I'm not implying people aren't able to have a stable clinical practice) but based on this phrase...
I'm also worried about the demand for psychologists in a healthcare-driven economy considering they cannot administer meds and LPCs and the like can do counseling.
Anyway, now that that's out of the way I'd like to hit on a couple other points. There was a PsyD on these forums a couple years ago who ended up going to a DO school (Nova, I believe). You could run a search to try to find out who that was since what I remembered is that he had some of the same reasoning you did.
With regards to how long it would take to do prerequisites is largely variable from individual to individual, but since you already graduated I would echo what was said about just taking the standard prerequisites. I wouldn't worry about piling on any of the tangentially-related courses like calculus and genetics. Some schools are requiring biochem but they are in the minority, by far. In any case, if you need to take a biochem course you could do so after being accepted, as it would just need to be done by matriculation. Obviously doing well in a biochem course can look great on an application, so don't get the wrong idea. Anyhow, since you're 26 you're likely not that far removed from undergrad (if at all). Taking any intro courses would be unnecessary. To give perspective, I graduated high school with a 2.4, did a light semester at the CC and got a 2.7 then didn't return to school for two years. I was able to complete two semesters of physics over the summer (since they do two summer sessions), then did my year of general chemistry and biology over the next year then finished off with organic chemistry the next summer. Obviously different things change by circumstance, but what I'm trying to highlight is that you don't need to pick up full-time enrollment in undergrad and take cell bio, physiology, microbiology, etc. in order to get into medical school. If you hang around the pre-med forums you may get this impression.
I had originally looked into medical school because of an interest in psychiatry. I chose the medical school path for psychiatry over the psychology route for many reasons. It's difficult to get an appreciation for the differences between the fields when first looking at them. Despite my naive decision, it took quite a while to begin to understand all the ways in which the fields differ. I'm glad I did choose the medical school route for many reasons which I will explain later, but it's imperative to understand that psychiatry isn't clinical psychology with Rx.
When I originally started the premed classes I didn't really enjoy them. I was just doing it to knock out the requirements (but then again it's not like people are taking organic chemistry for fun). What I really enjoyed were my psychology courses. When I registered for a neuroscience course from the neuro department (I was a psych major and figured I'd just take it for fun since it wasn't required) it was actually pretty fascinating to me to see how what I had studied previously became relevant within a psychological setting. I've found this to be equally true with the medical school curriculum (anatomy sucks). You may like medicine and you may not. You may end up loving psychiatry and you very well may end up loving radiology, or surgery, or pediatrics. Of all the military medical psychologists who ended up going to medical school (~4-5) I think all of them ended up in different fields than psychiatry (I'm only assuming they went to school with the assumption they'd do psychiatry).
I'm pretty naive on some of the intricaces of the PsyD so maybe none of what I say will be relevant. I know they're shorter than their PhD counterparts, but what kind of time-frame are you looking at -- 4-5 years? Again, I know a lot of the PhDs are funded with a stipend, but I'm not sure this is true for the PsyD -- perhaps you could clarify. If you're paying for the PsyD yourself, obviously there may be more motivation to quit and changes if called for, but if its funded then perhaps it makes it a more difficult situation.
Is leaving the PsyD and starting premed what your plan would be or are you thinking you could pursue both? I don't know if your PsyD allows you to do anything outside of the program, but ideally I'd try to take a premed course concurrently (like general bio). If you've got summers off (I have no idea) then that can give you a much better idea of whether or not you want to pursue medical school without having to make the permanent decision of leaving the PsyD. Others have mentioned doing the PsyD followed by NP/PA. I'd take a hard look at what your goals are and what you think you may enjoy and
why before deciding mid-level vs. psychiatry. I'd strongly suggest you contact your school's psychiatry department if they have one. Tell them your thoughts and see if they can get you some exposure to a broad range of psychiatric practice (inpatient, outpatient, ER, consult, child, etc.)
I'm sure there were things I could have written better but hopefully I was able to accurately covey some of my thoughts.