Building, and then maintaining, strength... Question

So I've been counting calories and working out for the past couple of months. Lost 15 pounds with only 5-15 more depending on how i feel, etc.... Anyway, I have a fitness question about building and maintaining strength.
A couple of months ago, I was a big dough ball and have spent most of my exercise time on bodyweight/strength/muscle building stuff and not much cardio. I feel so awesome to be able to actually do a pushup now (couldn't a month ago) LOL! I know that sounds like baby stuff, but its something for me. So as I get stronger, I keep wanting to get even stronger and more toned and am motivated now to do like 5 more this or 5 more that throughout the day. Which is great. But I also know myself and I know that in the long run, I'll be able to stick with my 15 minute exercises most days, with some longer days thrown in on the weekends and such, but probably won't keep the same motivation to exercise much longer as i have now. Long story short, I'm motivated now, but know that I won't be spending an hour a day exercising in the long term. Now I'll get to my question... what happens to the body when you taper down from 1/2 to 1 hour a day to a long term lighter exercise level? Like, if I'm motivated now, will I get strong/tone and then just lose it if I don't maintain the same level of exercise? Or can I use my current motivation to get me to a basic level of strength/fitness and then just maintain with maybe not so much gung-ho-ness? Does that make sense? I guess I was thinking this morning ... why are you doing an extra 15 minutes, cause if you don't keep working out at this level (which I know myself and probably won't - will do some everyday, but not an hour) then you're just going to lose it all.... At least that's what my little devil in my head was telling me. And then the little angel says... no, no, keep going, you'll get stronger now and you can maintain your strength later.... So, can anyone tell me which it is? or give guidance? Thanks so much in advance!