Shagging is just another form of exercise - the only way to increase your shag-fitness is to train specifically for it.
When I started punting I was in pretty good shape for my age - completed my first Olympic distance triathlon this year, not fast, but completed it no problem - with all that swimming/cycling/running I would say I have pretty good all-round fitness.
And yet my first couple of punts - around May / June this year (1 hr each) - after lots of kissing / oral / 69 - the "shagging" bit was probably 15-20 mins and I found it exhausting. Clearly uses very different muscles / fitness.
After "training" pretty much twice a week
I am now almost exclusively booking 2 hr punts and the shagging is probably a good hour of it. Did my first 3 hour punt with one of my regulars recently and it seemed like almost non-stop shagging!
A few tips.
1) It's not a race. Start slow and steady in each position, build up steam, then back off again to catch your breath. Repeat.
2) Mix up active and passive positions - alternate mish for example with cowgirl where she does most of the work.
3) Some positions you can readily take turns doing the work - e.g. doggie - if you get tired just stop and make her take the strain for a while. It's a nice feeling just kneeling / standing and letting her fuck you.
4) Or just switch back to 69 or a BJ for a while to take a breather before going again.