I've been rebuilding my fitness since back surgery. I got back on the bike on 12/29 and have ridden about 650 miles since then. Most of the rides have been on the cross bike on mostly gravel about an hour at a time. I'm almost to the point where I'll feel good about doing intervals. The gravel route is broken up into 6 sections, the longest being 3.6 miles.
Today I did a little more than an hour (22 miles) on the road bike, all small ring concentrating on my form. My back issues had twisted me a little because I was overcompensating for a weak left leg. Now I'm straight and I replaced my cleats to make sure the previous wear didn't force my feet back into a bad alignment.
Each workday I do exercise ball sit ups, usually three sets of 50. I was doing bridges to strengthen my back, but I've started adding push ups that I begin in the up position with my back straight for 30 seconds before doing a set of 12. I do 5 sets.
I don't do any weights, I have magical genetics that cause me to build muscle quickly and any additional muscle that doesn't support my core and help propel me down the road is just extra. Even the push ups are bulking me a little too much.