It could depend a lot on your training between now and then! When I tried to guess what I'd manage for 26 miles, barely able to run for 1 mile, I just multiplied up my 1 mile time, assuming that my stamina would improve.
If you're running less than 10k at the moment, there's a rough rule of thumb that doubling distance will mean 1 min per mile extra - so if you can do 5k at 10 min miles, you should aim for 11 min miles for 10k at the same level of fittness.