First, I admit that I am on the small side. However I have had tents (like the micro-zoid) equally small so had proportional entry and exit restriction.

We all seem to do some kind of conditioning to stay in shape- bicycle rides, running, weight lifting, etc. What is often forgotten is agility training (flexibility and balance). I try to keep my agility training fun. Indoor rock climbing is my favorite. Yoga, dance and other fun activities help too. Even every day activities help- gardening (squatting), scrubbing floors, caring for a little "rug rat" or toddler (lots of play with the grandkids!). As you build muscle, you have to counteract with some kind of flexibility exercise. The older I get, the more important flexibility and balance has become (and the harder I have to work on it!)

Taking off some girth also helps- both with walking and living on the ground. Although I know several rather chunky people who are surprisingly flexible.

One advantage of longer backpack trips, is that living on the ground and getting in and out of a small tent gets easier each day, so that the end of the trip, it is no problem. Since I get up and pee at night once or twice, I get extra practice! I suppose you could live on the floor in your house a few weeks before backpacking - remove all chairs!

Sometimes I will place a medium size rock where I can push on it with my hand when I get out of the tent. I also put my trekking poles on the side of the tent and can use them if needed. I too use a sit-pad as a kneeling pad when I get in the tent.

If I were tall I probably would use the tepee style tents.