In reply to Neil Williams:
I agree with everything Neil says especially the first sentence.
But I've done something similar to what you are proposing. My longest run until mid July was 8m tempo; I only ran 20 miles in the whole of August as I was focusing on some audaxes; I then went back to running in September and did a 10k race in the first week. Did a flat 11miler in mid September; 12.4 a fortnight later, and tomorrow another fortnight on I am planning to do a training half. (so approx 5 week period)
I just found the longer runs were a bit tiring and I needed a couple of rest days after but I think the key thing is not to add too much mileage or worry about speed. You should be ok if you have the base from 10k training.
Am also considering doing Conwy - looks like a nice race in one of my favourite places.