It's a bit early to start worrying about this, but then I am very efficient and I like to get worrying and that sort of thing dealt with early so that it doesn't take me by surprise.
The thing is, in my thinner days I was strong but I had no stamina. I used to be able to work hard and carry things and dig the garden and whatever, but only for a limited time. And once I had to stop and rest, I wasn't able to do much for days afterwards. I never was able to build up long-term strength, however fit and well I felt. It was like that even when I was a child - I used to look around at other children and wonder why they were so much stronger that I was, when I knew that my diet was much better than most of theirs. We always had very good nutritious food and although I didn't eat much, since we never had puddings and suchlike, nor convenience or processed foods, everything I ate was good for me. Some people seemed to live on spaghetti hoops and Mars bars, but they seemed stronger than I was - I used to get headaches and was often tired, although I was not unhealthy.
The thing is, however much exercise I took, it never seemed to build up my strength, only use it up. So one of the consolations for me of putting on weight was that I had much more stamina. Nowadays, I can keep going for a long time, and the next day I'm ready to go again. It's possible that I pace myself better, but I think there's more to it than that from past experience and I wonder if I'll start to flag quickly once I'm appreciably thinner.
The reason I wonder this is that I've been cycling since November and I'm not at all stronger than I was several months ago. Sure, it took me a few weeks to manage some hills I now don't think anything of, but any improvement in that respect was made in the first three or four months. I thought it would all be more pleasant by now, but there are a lot of hills (such as up to the swimming pool) that I don't think I'll ever be able to cycle. I haven't found the limit of my comfortable distance yet and my legs never ache afterwards, but I don't get better at it or more energetic overall. I can't help wondering that as my fitness (presumably) increases and my weight goes down if the one counteracts the other. That is, that I'm not actually too good at building up any muscular strength nor, more importantly, staying power.
I don't suppose it'll be anything to worry about for at least another year, but it's been on my mind for a while.