I would really recommend doing a half as it will give you a good indication of what you need to work on. There are two approaches you can take towards the half marathon before running a marathon. You can either view it as a long training run and run it at your marathon race pace, or you can run faster than marathon race pace and try to go for a PB.
If you are trying to run a PB and let’s assume that your half marathon time is quicker than your marathon pace, then it is important that you have been running at that pace in training. The speeds at which you run in training are the speeds that you are going to get better at running. If you go out the door 4 times a week and run 8 mins per mile for every mile, you are going to get better at running 8 minute miles for a longer period of time.
There is a time and a place to run slow (recovery runs) and there is a time to run fast(er) - during sessions/workouts.
This is why you shouldn't always run at one pace in training and expect to get quicker. You need to work on the pace that you want to race at. This doesn't mean doing every single run at race pace, it means organising your training sessions so that you are working at different paces. There is a time and a place to run slow (recovery runs) and there is a time to run fast(er) - during sessions/workouts.
The first thing you need to do is work out what time you want to run for the half marathon and find out how fast that is per mile, and then per 400m. Let's say you want to run 90 minutes for the half marathon. This would mean you would need to run every mile at about 6:42 and that would be a 400m pace of 103 seconds.
In training for a half marathon, the shortest distance you need to run is 400m and there would need to be a lot of them off a certain amount of recovery time. A session I have done before is 12 x 400m with a minute rest between each one. You should start off running each 400m at the pace at which you want to run your half marathon and as you get through them (if you can) speed up the reps. This means that you are getting the legs used to running at a pace, albeit for a short distance, that you want to run your half marathon at. You can tweak the session depending on how you find it; you can extend the recovery slightly, run each rep slightly quicker, or if you're feeling REALLY fit you can shorten the recovery to 45 seconds!
The fitter you get the longer the distances you'll be able to run at a slightly quicker pace.
The fitter you get the longer the distances you'll be able to run at a slightly quicker pace. A session you should look to aim towards is 6-8 x 1 mile with 2 minutes recovery and doing each of the mile reps quicker than 6:52 per mile. The more training you do at this pace the better you will get, and you'll start to feel the sessions become easier, and that is when you'll need to adjust the times/recovery. Remember as well that all this 'speed' training will get you running quicker, but when it comes to the marathon you'll need the strength as well to keep going, so don't neglect your long runs! Remember you can't train for every event under the sun and this is why it is likely you won't run a 5km PB in the build up to a marathon.