tadawdy

Probably not eating enough. Cutting calories and optimum sports nutrition (for performance and recovery) are not best friends. It's usually said that a caloric deficit of 1000kcal/day is the max you should consistently do. So...if your RMR is about 1500 kcal, and you do 700 kcal of exercise, and add in a few hundred lifestyle calories, you're right there. Anything more and you're pushing it.

I'd agree that you probably just aren't fueling well for your rides, which is admittedly a tricky proposition for someone whose main goal is to lose weight. If you take basic recommendations and eat 20-25 g protein with 80-100 g carbs (4:1 ratio) after a good, hard workout, that's 400-500kcal right there. And a 500 kcal deficit is the lower limit of the typical dieter's target range. In some ways, you're robbing Peter to pay Paul.

I don't know if your diet is low-carb, but that will certainly make you feel tired on a long ride. If you're not eating them during (like in a sports drink), your performance will suffer. If you don't eat them after, you will not recover as well.

Cycling isn't an activity that should regularly leave your muscles sore. Unlike running or weightlifting, there isn't much of an eccentric phase in cycling. This is the contracting but still lengthening phase, and is much of what makes you sore afterward. Ever hiked? It's the downhill part that kills your legs.

As for using a smaller gear: that isn't necessarily bad. Most of us started out grinding a big gear because we thought it was pro, and now spin at a more reasonable cadence to place more load on our hearts. What cadence are you using now? Are you going the same speed or faster than you were?

Unfortunately some of that 40 lb you've lost is lean mass, so it certainly wouldn't be surprising if you were stronger in absolute terms before the weight loss. In relative terms, which are often more important to cycling (think W/kg, VO2 max), you should be better now. Weight loss is made worse if you lose the weight very quickly (though a couple pounds a week is considered reasonable for an overweight man). It is also made worse if you were doing no strength training during your weight loss.

Another possibility is that you are overtrained to some degree. If you've been really ambitious with your diet and exercise this whole time (40 lb at 2lb/wk is about 5 months) without any breaks, it might be time for a little R&R. This is just a possibility, food for thought. Try looking at your diet first, and see if some additional calories (carbs?) are in order.