I'd add weight training of some kind to your walking/ yoga routine. Maybe a class twice or three times a week. You will need to start eating more if you want to build muscle. So you might want to consider diet as well as meal timing. By adding lean protein and healthy fats you'll regain energy.
Feeling faint might be a low blood sugar thing vs. low blood pressure. This can happen when you're not taking enough calories in or not the right kind for your energy needs.
I'd add a class with weights a couple of times a week to start. It will motivate you to do a kind of exercise that won't be the easiest for you. (Most ectomorphs prefer cardio but it won't help you build muscle.)
I'm spotty right now with all exercise (because life has been insane the past year) but intend to get back onto my routine as soon as I have finished oral surgery. (Ugh).
My preferred pattern is an average of 10,000 steps a day (sometimes more sometimes less) and 4 to 5 actual "workouts" -- weights at least twice and HIIT about twice as well. I sometimes do yoga but have been off that for a while.