When working out (or restarting workouts), it's best to start at a light, easy to do level, and then as you get to the point where you can do it without much strain, GRADUALLY increase what you are doing. The thing that discourages a lot of people from working out is trying to do too much at the beginning, getting sore, and then thinking it's not worth it.
When I first started getting back on my bicycle (at almost 50 years old), I didn't think I was the young stud I was in my 20's and 30's, where I was biking 50 to 100 miles per day. I started out at around 4 miles, and gradually built myself up to where I could ride 20 miles and not even bat an eye. Sadly, the long haul rides I used to take when I was younger are not something I do anymore. Used to be that almost every Saturday when I was stationed in Jacksonville FL, I would ride 42 miles from Orange Park out to Jacksonville beach, probably ride 5 miles while at the beach, looking at all the pretty ladies, and then back the 42 miles to home. I'd leave sometime around 10 am and was back home by around 4 or 6, depending on how much time I spent at the beach.