Well, the first thing I would suggest, if you're 50 lbs. overweight and middle-aged, is don't assume there are no medical conditions. Why? Because 90% of losing weight is the food you eat, not your exercise regimen. Exercise to feel healthy and fit, not to lose weight. If your blood sugar spikes after a typical carb-laden meal, then the "normal" low fat, high carb diet will impede your progress at every turn. But you really won't know that unless you test yourself. if your blood sugar doesn't go much above 140 1 hour and 2 hours after a typical meal, then all you need to worry about is calories and cutting back on them. But if it does, then you should look into a low-carb high fat regimen that will keep your blood sugar levels within a normal range.
No matter what diet (and I mean that term in the sense of a life-long way of eating, not a short-term fix) suits you, starving yourself is a surefire way to sabotage your long term weight loss. Yes, you may shed some pounds quickly, but the odds of keeping it off are stacked against you. Not only that, but if your calories in drop too low, your body will think you're starving and do everything it can to conserve fat.
As for building muscle, that is a slow and difficult process, and slower and more difficult the more "middle aged" you are. You need testosterone to build muscle, and as you age, you manufacture less and less of it. That said, many people make the mistake of thinking that muscles develop while being stressed, when actually they develop as a result of repairing themselves after stress. If you never give yourself time to recover, that repair process can't take place.
If you're short on time, you can find a lot of useful information here and elsewhere on interval training. As long as your heart is healthy, this is a great way to achieve fitness without spending hours and hours in the saddle.
At the same time, I love to ride long distances, and the only way to get there is to ride. When I started cycling, a 25 mile ride was a long ways; now it's just a warm-up. I got there by riding a mix of routes. One day a week, I ride a relatively flat course, with only around 1,000 feet of climbing and lots of sprints. A second day, I ride a more hilly course, with 2,500 feet of climbing. And on the weekends, I do a longer ride (65 to 90 miles) that's a combination of both, with around 3,000 feet of climbing. I don't ride two days in a row, mainly because my older body can't take it. If you're in your 40's though, that probably won't be a problem, as long as your "recovery rides" don't stress your legs too much.
This is about as generic as one can get, I imagine. You can find lots of specific training regimens on various cycling web sites, including this one.