how to improve my speed?

WHY?

You are a road runner. Do you want to abandon that for track? These are different disciplines requiring different shoes and different foot striking. Do you want to get down to 60 or 55 or 50? Do you want to be competitive in masters track and, if so, at what distance? Did you run in h.s. or college? If so, what?

Your 400m time is about what should be expected for your 5k and marathon times. What is your 100m time?

Just want to see how fast I can run. I did not run in high school or college - only picked up running as a hobby after college. I don't have any official PRs in distances less than the 5K.

Track is a different animal from road running. Speed is generally thought of as either top end speed or speed endurance. It's easy enough to find material on the internet on both. Given your interest in the 400 and your lack of track competition, speed endurance is your aim to get down that 65 400 to, say, 60. Only running 400s on a track will not get you much better, if at all. The 400 must be bracketed with 200s, 300s and 500s. You will have to do repeats at these various distances and I favor getting these repeats above 90% of maximum effort later in your first year of this.

You would have to abandon road running altogether to get a sense of how fast you can run a 400 and by 33-36 you would know. I doubt you will ever know if you are a "hobby" track runner, although this is not meant to discourage you from trying.

Basic equipment means track flats. You should be gradually moving from heel striking to a more forward foot strike. Everyone has fast twitch muscle fibers and you have got to use them in 200s and 300s, and be running close to your fastest, taking as much time as needed for recovery between runs. This is a different approach than would be used in h.s. or college varsity training.

I worked out-of-town for a few years and the running trails were not the best. But I did run at the university stadium. I did hills on the stadium ramp...it was very steep and about 150 meters. I ran up at full speep with perhaps 3-minute recoveries. I worked up to about 30, though you really would be fine with as few as 10. This did wonders for my sprinting ability. You could also play pick-up basketball. In college I played pick-up games for like up to 8 hours a day. I never had as much speed as I did then.

I also do not do much short or middle distance, but I find it very useful and beneficial to my speed during my long races (5k, 10k, 10 miles) to do some speed work, preferably on the track. The workout varies depending on pace, but doing some speed just once a week is a good thing to do. I usually do one mile at a pace about 45 seconds more than my best mile, then a 1200 at a pace 30 seconds more than my best mile. Continue to move down from1200 to 800 to 400 to 2 200s to finish it off, with your 200s at your best mile pace. Really get into that sprinters form at the last 100m of your 200s. You can do a slow 400m jog in between each set. To increase the difficulty of the workout, you can do a shorter recovery. Hope this helps!

Start with 3x20m sprints all out. Over a period of three months, build up number of reps and distance to 10x200 fast. Run these speed session twice a week and adapt the rest of your training accordingly. This will improve your basic speed substantially. Maintain your speed for the rest of your career with some 200's and striders every week.

If you want to run a fast 400 you need to work on your "anaerobic" system with reps up to 600 m. Do this after you've built your basic speed. Keep in mind though, that such training is unlikely to contribute much to any improvement in longer distances.

OP: before you ran that 5k PR what types of workouts were you doing?

i think you could drop your mile/5k PRs just by adding fartleks and a little track work. it depends on your goals.

i'd recommend dropping mileage to 60-70 and doing 2 at first, then 3 speed workouts per week. a track day, a fartlek day, and a short tempo day (20min tempo.

I did do some track workout before my 5K PR. The one pure speed work I did was 10 x 400m with 5 minutes full recovery, at 1:12-1:13 per lap on average.

One thing that is weird about me is that I have these genetically massive leg muscles, and people see me and assume that I'm a sprinter not a distance runner, and they assume that I lift weights all the time but in fact I never do. I never had any sprinting speed. I am relatively good on running uphill, however. I'm not sure what that indicates.