Keep at it...my kids ask why I'm off riding...."to keep up with you two" I say...although I see my son will be up for rides in a few years - road bikes and all - he's been asking questions aged (a small) 7 about road bikes...... although he does beat the bigger kids in 'races' down the road easily..
I found/still find commuting at a hard fast pace is great, and on a slicked MTB..... get's your fitness up.
I won't say it's easy - I've ridden bikes all my life, and 'properly' from 16 in a club, so it's imprinted, but give it a year, you'll be getting there. Cycling isn't easy, no matter how fit you are.. you just push harder.... what was hard is now easy, but you are now doing a few mph more......and finding it just as hard.... use the local topography (hills) as there is no going back...I felt like getting off on a 1 in 5 today, for a millisecond, just as it kicked in... got up it no problem....other than a bit of swearing......as going back home the same way was as bad.... DON'T STOP ...DON'T GET OFF...
PS I'm 38.5 and now fitter than when I was racing in early 20's - just need a 10TT to prove it. Same weight, but no lightweight - 12 and a bit stone for 5 foot 9 inches - so no skinny bugger, but lean and mean (ish)