When I'm curious about a result, I look at my data. I have both power and HR on one bike, but only HR on the other. In either case, I look at time in zone to assess my fitness. Last Sunday, I was in zones 4/5 for 1:15 - I was really fresh. This Sunday I was OK, but not all that fresh and the total was about 1:00 with the same amount of Z5. That data tells me if I was in condition and well enough rested to make a serious effort. Without data, it's like "who knows?" These were both fairly hilly rides of about 3 hours. On rides of that length, I can usually manage about a .8 intensity number and have ridden to exhaustion. IME, that's a good way to get stronger. Of course one first has to have the aerobic conditioning to make that possible, so that's where to start: aerobic volume/week. I'm 75.
I wouldn't blame the weight gain too much. You can run your numbers through this calculator:
Bicycle Speed (Velocity) And Power Calculator
I suspect that 10 lbs. might cost you .3 mph on a long 10% climb