Them’s a lot of questions, Choosybeggar. First, I’d say that your base is a little shakey, at 10.5 miles a week to do any kind of speed work. I’d recommend building a better foundation first. Speed is not important in this phase of training. Visualize a pyramid. The bottom is your base and this is built up on distance only, with no regard to pace.
I’d recommend, to avoid injury, building your base to at least 15-20 miles a week. (Most runners who do speed work, do much more. I’m doing around 60 now, but I also plan to run a marathon in a few months.) Since you’re young, even younger than I was when I started running, you can do two speed works a week. Experts say one should consist of intervals and the other a tempo run. A tempo run is when you run close to your lactate threshold for about 20 minutes during a run. (See your link for “lactate threshold.”)
I’ve already discussed intervals, which differ from repetitions because they involve incomplete recovery during the recovery period and are not run all out, as repetitions are. No one does reps any more. Over 20 years ago, the experts found that intervals are better. You should try to run 12 X 400, which means running 400 meters 12 times with a 200 meter recovery jog in-between. However, since you are a rookie at speed work, I’d recommend that you begin with 6 X 400 at first, and gradually work up to 12.
Those who are more seriously involved in running then graduate to other stuff, such as ladders, reverse ladders, 800s, etc. For your purposes, 400 meters is fine.
NEVER, NEVER mix speed work with endurance. Check the pyramid again. Coaches and experts use the pyramid to peak for a certain race, the acme of the pyramid being the race date. As the pyramid gets narrower, your weekly mileage decreases, but your pace increases.
I don’t have a heart monitor and never have. You can check your HR manually after an interval. (Technically, the “interval” is the recovery period, but many use it to refer to the speed period, as I just did.) After a speed period, your HR should be quite high, perhaps 140-150. Check it again after the recovery jog, and if it is 100-110, then you can do a repeat. If it’s higher, wait until it drops below 110.
If you have any additional questions, I’d be glad to help if I can. You can email me if you wish, but I’m sure others will want to hop in and help too.