When I did a running course, I started from a bit above nothing (hence first weeks were boring) and ended at 5 km in 30 minutes (easily, with 8 min remaining). The course took 12 weeks to reach, approx 3 runs a week (so sometimes one extra day off). I was around 35 y.o. then, so it was still relatively easy to get in shape. If you're older, this is more difficult. But the course was nice because I had a (Belgian) woman in my ears telling me what to do. Really easy. You do what she tells you to. And if you can't, you still give it your best. Oh, and dear reader, do yourself a favor: do a proper warming up, every time. You don't want to retract a muscle.

You're probably just getting old and I don't think most people appreciate the effect that years of being relatively sedentary can have on you. I ran competitively in high school and stayed at it mostly recreationally, but not consistently, joined the Army at one point, then got badly injured in my 30s and didn't run for nearly a decade. Now I run again in my 40s, frankly more than I ever did before.

At age 12, I ran a 5:05 mile with no training except running a weekly all-out mile for PE class. At age 14, I ran a 17:14 5K after three months of training in my first cross-country meet of my first season. I PRed at 16:04 two years later. In my late 20, I was still hitting an 11:30 2-mile in the Army's fitness test pretty easily, mostly without running. I swam every day, did a lot of rowing, and ran track repeats for 8 weeks leading up to the test.

Today, I run almost 80 miles a week right now, a structured, principled program, and I can't get anywhere remotely close to the times I once used to run almost effortlessly. Same person, same genetics, but in my 40s after a decade of being mostly sedentary. It takes its toll and it makes people who take this up as a hobby because they got fat in their 30s think something is impossible that likely would have been quite doable if they'd been doing it their entire lives.