Crikey me, Nathalie! You're a speed merchant and no mistake. Very roughly, double your 10K time today and add 5 minutes for the extra 0.7 mile, giving you 1 hr 45 mins. just over 8-minutes a mile, and far from shabby

Now, you may be 5 minutes over or 5 minutes under that, depending on how you respond to more mileage. Rule of thumb's not to increase mileage by more than 10% a week. So, if you're running 20 miles a week, your next target is 22...and so on. Keep stretching and try swimming or gym as cross-training if you've time.
Many run fair HMs on 25-30 miles a week (peak); however, miles = stamina, a vital asset for distance running. As it's your first, aim to get a little over 30 miles a week for the last three weeks but one before the HM. In the last one, cut right back (taper) to less than 15 miles and you'll benefit on 4 Villages day. Good running!!