A quick calculation tells me I need to run at an average speed 12.65km/h to do 21.0975km (half marathon distance) in 100 minutes.
Put another way, I need to run a pace that will get me the following times:
1 km in 4 minutes 44 seconds
3 km in 14 minutes 14 seconds
5 km in 23 minutes 42 seconds
8 km in 37 minutes 55 seconds
10 km in 47 minutes 23 seconds
15 km in 71 minutes 5 seconds
I have only ever achieved the pace required once in a distance at or above 8km, in a really hard fartlek run I did almost exactly one month ago!! I have certainly improved my endurance and strength since then.
It looks like I have taken on a pretty difficult aim. So I need to work harder on my speed, and warm up carefully to avoid injuries.
The next race I have is a 15km run on 31st May, just over a week away. I have set a goal to do it in 71 minutes 5 seconds or better. Big goal, so lets see how I go with that. I usually run faster in the races I have done so far.
I have read that a good yardstick for the half marathon pace you can potentially do is the 10 km speed. To do a 100 minute half marathon, you need to be able to do a 10km run in 45 minutes flat. I don't think I have any runs in training that are 10 km long, I may have to make a small change to my schedule to get one in some time just to see how I go.