Here's what I'm talking about: T25 and P90X3.
1. If the people in the video are wearing shoes, wear shoes. Otherwise you are more likely to hurt yourself.
2. T25 is a lot more cardio. I used it to slim down and I think it's probably more helpful for us ladies. I switched to P90x when I needed to start building up muscles because my weight was inching toward my high school weight, which didn't seem healthy. I didn't want to gain inches, so I decided to build muscle instead.
3. Don't force yourself to do every exercise. Some of them will probably hurt you, and those won't be the same for everyone. Adapt. Many of the exercises will be safer for us older folks if we simply take some of the bounce out.
4. Don't force yourself to match the pace or endurance of the people on the video immediately. There isn't any point. Endurance and strength are built up gradually. If you can only do 30 seconds of a 60 second exercise, then do your 30 seconds. Gradually, your endurance will increase. If you are planning on sticking with program for the full 70 or 90 days, it's kind of silly to push yourself to exhaustion rather than work up to what you want. So, yes, push yourself. But not so much that you give up or that you hurt yourself. You have time. If you are sweating or stretching, you're making progress. So relax.
5. Discard particular workouts. I'm not going to do something that focuses entirely on abs. As near as I can tell, I have none. And a workout with lots of push-ups and pull-ups is pointless for me. I can't to do them. The workout ends up being me attempting to keep up, not being able to, and doing lots of standing around waiting for them to get to the next exercise which -- surprise! -- I can't do either.
6. Deliberately and consciously enjoy the happy endorphins afterward. I love having a better mood and (usually) more energy for the rest of the morning (or sometimes the rest of the day). Keeps you coming back for more!