Weight and strength will come with time. Done every other day, then 2 days off. If you can't do that weight then try it again next week. If you still can't do it the next week, then you've hit a temporary plateau. But if your legs are still sore from the last leg work out, take at least another day off.
Rest is when the muscle grows.
This is usually taken by pressing on the front, left side of the neck, or the inner wrist, and counting the number of beats in a certain time. Your MHR is defined as 220-Your Age. This is your Training Zone. So a 20-year old should keep their HR between 100 and 160. Keeping the HR above this zone will not provide additional aerobic benefit, in fact, it may provide no aerobic benefit at all.