After my last injury (I’m 40 as well), my physical therapist said load management is the key to avoiding further injury. What that means is: Load is both running volume (miles/kms per week) and speed (avg number of faster runs). Don’t increase either drastically. I doubled my volume in 3 weeks and added 2 speedy workouts per week, resulting in injury. You can test your 5k time but I’d recommend starting out slower than you like and also add some body weight training to your routine.

Every Saturday morning at 9am at parkrun

Not often. 1-2 times before your race. Never test too close to race-day. More testing ≈ higher chance of injury. Since you just started running, and depending on the time range of your goal, you should focus on obtaining a strong foundational base by doing 8-10Ks and move backwards to the 5K. Try doing something like this: 4K @ Z2 4 x 400m @ threshold 100m walking rest in between Keep the watch running throughout the entire session.

A good start (once you’ve built a base) would be to do some speed intervals as goal race pace. If you can’t do 10 x 400m at race pace - it’s unlikely you’ll do 5k at that pace. I wouldn’t race it until then.

You should do a 5k training plan for 12 weeks and then run a race at the distance. There’s very little training benefit to doing a time trial during training as the recovery would require time off training (and you’ll have training stress so you won’t be as fast as you’re capable of anyway). Also, if you do it it too much, you’ll probably hurt yourself. You’ll get faster by consistently running and to do that you need to run slightly slower than you are capable of. Sub 20 is *very* ambitious…

Are you already a runner at all? If not be careful. If you can already run a 5k slowly you’ll have no problems. It’s not the speed that will injure you, it’s unprecedented volume. Here is what I do to get back to sub 20 5k: Run 4 or 5 days a week. One long run (10-13mi) One tempo run (5-6mi) One speed day (eg. 7x400m) One or two slow recovery runs (3mi) Here is my 5k program offset for races and benchmarks every other Wednesday. I just turned 40 last week.

I wouldn’t do a full-gas 5k effort more than once a month, not because of injury, but just because you’re unlikely to get 100% more often than that without excessively disrupting your training rhythm. There’s nothing wrong with running a fast 5k every week (a lot of people do this at parkrun every Saturday) but it will be a way off your full potential if you’re doing it during normal training weeks.

You don't test your time. Mark out where 5k is on your routes, and when you reach half way, ask yourself if it's feeling good. If it is, go for it. My fastest was 22.23 when I was 55 years old.

Train your base first. Lots of easy and slow mileage with workouts being rare. After a few weeks, start incorporating interval training/tempo runs/fartleks. After a few weeks of that, start a taper, which means cutting back on your mileage a little every week so that you can rest your body to be more ready to race. Make sure you incorporate more high speed/low volume training like fast 400m repeats during this taper period; you can also race weekly without issue, but only for a couple months.

With a few years I don’t think that’s a bad goal, given your dad’s achievements. Just don’t think you will do it in your first season. But the answer to your question is simple.. depending on where you live (which defines the length of your season), start entering 5K races. Don’t enter more than one per month. It’ll take most of a week to recover at first from hard 5K race so racing more than about once/month means you won’t be training as consistently as you’d like.