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Alot depends on your age, weight and current competence. A bit of background may help establish a starting point for you......
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Not bad times at all for your age and weight Jonathan. I'm guessing that your weekly mileage tops out at around 30 miles?
The best benefits to your race times will come from increasing your weekly mileage. I'd aim to have one long run each week, working up to 12 miles at an easy pace and perhaps up to two quality sessions involving some speedier rep work on one of the days and perhaps a 5 mile tempo run on the other (I wouldn't advise doing these without at least one rest day/easy day in between). Whatever other days you might have free each week would be best served just doing easy paced running to keep your aerobic system in check.
On the 10K front, I'm guessing that 40 mins will be your next target. Being that you expect to be faster than your previous 43 mins. Tempo runs for someone not far off a 40 min 10k would probably be in the region of 6:35-6:45 pace per mile. On race day, you'd be looking at 6:24 per mile or thereabouts.
I've been known to work up to 10 x 800m at race pace with 1:30 recovery. Then 5 x mile reps at race pace with 1:30 recovery. Then 5k at race pace continuous. Lastly 4 miles at race pace continuous.
If I can do 4 miles continous at race pace in training, I can usually manage the 6.21 miles on race day.
As for tempo runs, they shouldn't be at race pace, 10-20 secs per mile slower usually. Much more comfortable than race pace.
I've never raced a 5K (we only have two every year, one is on track, the other is part of a 4 man relay and I've never got round to them) but my 10K is 36:53. The 5Ks that I've done in training are all 10K race pace time trials, but my best was 18:10 when I was a few seconds faster each mile than I needed to be. I'd like to think that I could shave at least 30 seconds off this in a race.
As for my training, the intervals don't stress like a real race. I'd go so far as to say the 800s at 10K race pace are quite easy because you have a 1:30 break every rep, plus you're only doing 5 miles equivalent. Its a great way of getting the body used to the pace without leaving you out of breath.
The time trials are tough though!!! But I only do one a piece in the final fortnight before race day.
How fast do you do your 10 mile weekly runs?
A person's weight has quite a bearing on race times. I'm a couple of pounds under 11 stone at the moment. For your weight at 12.5 stone your times are excellent.....and increasing mileage a touch will see your weight drop too. I was around the 12 stone mark when I was doing more weights and gym work......and whilst I was probably quite fit in one way and didn't have much in the way of body fat, theres no hope that I'd have cracked 40 mins over 10K. I had to be around 11:5 to do that. You may well crack 40 mins at a higher weight than 11:5 which speaks volumes for your fitness. Granted, height, technique and genetics have quite a bearing too......
As for your 800s, you might be running them a touch fast. There are pace intervals and speed reps. I've read a book recently (The Running Formula by Jack Daniels PHD). The book recommends the optimum paces that your should be running your reps based on a current race competence. Naturally the recommended speed reps are quicker than the recommended interval reps.....but the book lists several rep distances. In paperback it might be a good read, I didn't pay much for my copy!.
As for your 10 mile runs I think your pace is right on the money at around or slightly quicker than 8 min/mile pace. I'll bet you're not heaving at this pace? Perhaps an extra mile or two at this pace wouldn't hurt at all just once a week?