First of all, you're describing separate careers--you probably wind up having to pick environmental research/advocacy OR entrepreneurship. And I'm not sure that an MBA really helps with your second career path. While an MBA could theoretically help you if you're in administration and running a hospital or healthcare system, from my perspective successful entrepreneurship comes down to being really good at innovative discovery research. Find something that works and that nobody else has come up with, and you'll figure out a way to market it. In fact, major academic medical centers have whole departments dedicated to figuring out how to license intellectual property to companies or spin off your own start up, so I wouldn't worry about gaining those skills on your own.
I can't speak to environmental research/advocacy, but again being successful in entrepreneurship comes down to being really good at basic science research. If that is TRULY your passion... then I'd seriously consider getting an MD/PhD. To hop on the treadmill of academic medicine you need to get grants to fund your lab, and to get grants you need publications/preliminary data, and to get THOSE you need protected time where you don't need to be in clinic seeing patients. You CAN get that time later during a heme/onc fellowship, and in some respects that makes more financial sense because you're going to get paid for those years as a fellow rather than as a grad student; the downside is that you're not going to be fully protected as a fellow, as you will undoubtedly still have some clinical responsibilities, and not every heme/onc fellowship is geared towards training physician scientists. If you come into fellowship with your pubs from your PhD already banked, you're just going to be at a leg up on all the other fellows who are starting from scratch in generating data.
If getting a PhD is not appealing to you, then I would start networking. There are a number of competitive but high-profile fellowships which can serve as a launching board for an academic career. Two which are well-regarded and have threads on the first page of this forum are the
NIH MRSP and the
MSKCC summer fellowship. Getting those kinds of experiences on your CV will definitely help you stand out when you're applying for the kinds of IM residencies that can then feed to the heme/onc fellowships that produce the leaders in the field.
Again... not saying that anyone NEEDS to do any of this. You can certainly just "do well" in med school and find your way to a solid residency and then a solid fellowship and then figure out the academic game at that point. But you're going to need to get that protected lab time at some point, either early in your career or later in your training. If you seriously are aiming for the endgame where you're at a major academic medical center generating novel and patentable lung cancer therapeutics... then I would consider these options.
Source: I'm an academic peds heme/onc.