I will share my story here since I was mentioned as it may help a lot of you. I have always done sports like swimming and biking, oh and I am very tall and skinny. When I was around 20 I fall into the fiat trap of triathlon, well the responsability is mine but the training of biking and running was basically on flat terrain I was also going to the gym. I ended up destroying my leg mechanics, part of training only on flat and also overtraining. In the last years I was hopeless about getting fit again as I was in my 20's, out of hope I just restarted biking small distances like 20-40km on hills at natural speed, I have tried it before but never for long months before, now my mechanics are getting back to normal and start to gain some tonus and more muscle again. After training for thiathlon a few years i lost a lot of tonus, as if my muscles were not holding my bones correctly which gave some pain and the mechanical problem. My lesson is don't listen much the experts, you can do what's natural and a pleasure for you. IMO for those that like biking and running hills can be a substitute to the gym, but it's a real proof of work.