I feel like this is more of a problem for techs than doctors. As a doctor, you can sit down at a computer or desk and do things like write your notes in between appointments, or make phone calls or clients to report the status of their animal. There's plenty of default "breaks" in the day to rest your legs. Yes, you are expected to be up when needed but you also have certain obligations in which you can obviously take a seat and breathe for a minute. Thus the appeal [for me at least] of being a doctor..... techs do not get that luxury, as it is usually seen as wasting time.
This depends a lot on the clinic, but I wouldn't necessarily assume it to be the case. Of my four jobs as a veterinarian:
1) Had my own desk, but there was no telephone or computer. I did get to sit down a good bit on slow days (to write records, look at labwork, look things up in textbooks, etc), but on busy days I typically didn't have time to walk over to my desk and therefore did all these things standing at the counter. Phone calls, VIN research, etc were all done standing at the pharmacy computer.
2) Nine doctor practice, TWO sit-down workstations. When a colleague and I found a stool in the kennel and set it in front of one of the standing workstations, thus making a third seated workstation, the boss became angry and took the chair away, saying "I'm not paying you to sit and write records - I'm paying you to be in rooms making money." So there were occasional times that I could snag one of the sitting workstations, but it wasn't often.
3) VERY slow hospital.... I had my own desk/office and plenty of time to sit down. But, on the downside, my pay was cut 40% after a year of employment because the clinic was financially failing!! Needless to say, I had to start job-hunting with that sort of pay cut. (And no, my boss did not make any corresponding cut to my hours... I was expected to work the same number of hours for 40% less pay.)
4) Current job: There is a sit-down workstation for each doctor, but we are typically too busy to sit. When I come out of an exam room to write a record, I typically stand at the counter to do it (not taking the time to pull out a stool), because there's already another room waiting for me. If I'm sitting, it's because we've had a cancellation/no-show or because it's after closing and I'm doing records/call-backs/etc.
So, that's not to say that there aren't vet jobs where you get relaxed time to sit, call owners, review lab results, etc... but if that's something important to you (or anyone else reading this thread), you'll definitely want to pay attention and watch for that during your working interviews. I definitely wouldn't assume that it will be the case.