At 70, tilting your bike is no big deal. Just do it. As for stretching, that is a bit complicated. There are books on stretching for cyclists and some info on the web. If I were you, I'd join a gym/health club with stretching/Pilates/yoga classes, and take about 3 classes a week. At your age, a regular regimen of stretching, balance and resistance training would be very useful - for the rest of your life. I'm in my 50's, and take these classes, and need to start a more active program of resistance training. I plan to do these things the rest of my life.
I ride with a guy who is 82, and I think all he does is ride - a lot. But he is exceptional.
Back to putting a leg over your bike... This is actually a fairly complicated process. It involves balance and hip and leg flexibility. Pretty cool stuff to be good at as we get older. As I think about it, a ballet class would probably work pretty well too. I've never looked for ballet classes for older people like us, but the concept of training for controlled, fluid movement makes a lot of sense.