Since I served in a foreign country with amenities inferior to America, issues always arose with diet and health. And I was in a relatively stable country. I can't imagine serving a mission in Africa where you spend half your time throwing up and having shivering fits from Malaria. I'm not making this up, I have never felt so unhealthy, overworked, overstressed, and simply tired on my mission(especially towards the end). I am not the exception. We used to compare before and after pictures of missionaries and laugh at the complexion on our faces after 2 years. Once again, I served overseas, so that's just my experience.
There are no special privelages. You have to be with your partner at all times. You have to wake up at 6:30(sometimes you can wake up early, but like I said, you are always tired, and if you choose to wake up early, your partner has to wake up early as well. Imagine telling some average Joe that you expect him to wake up at 5am everyday so you can selfishly run your 10 miles. Not a great way to get along. And you are with them 24/7 so getting along is absolutely essential).
You have a set of rules, and you choose to keep them or not. No one is 100% perfect. Yes, you could simply mail it in, break the mission rules by running on your own, structure your missionary schedule around 10 milers,and make the 2 years of missionary service pseudo training, but that's not why people go on missions. People don't serve missions to run and train.
If you do your mission right you simply don't worry about things like this. Nearly all your thoughts and efforts are directed towards helping others. I'd get calls from people all day like "I want to divorce my husband" or "I lost my job and can't pay the heating...I have 3 little kids..." or "my husband just drank too much and got in a car crash and almost died.." You name it. And you are expected to help and give support through all of this with your 20 year old experience. So as you can imagine, you tend to be busy and your priorities lie elsewhere.
Priorities aside, the system doesn't even allow for training.
Training as a 22 year old BYU Sophomore that hasn't run in 2 years as opposed to a 20 year old Stanford Junior that hasn't missed a day running for the past 6 years is NOT an advantage. You'd be surprised how past your prime you feel.
If there are other questions, feel free to ask.