I got 16 miles under my belt yesterday as well. I didn't feel too bad but it was 45 secs a mile slower than (hopeful) MP. 9 weeks to go?
PG3 - don't fret about long run pace too much, so long as you are getting the pacey stuff in somewhere in the week. I did London in 2:54 last year, but my long runs averaged over 8 minutes per mile and none were within 45 seconds of MP. I would say mix them up a little, some steady & comfortable and one or two faster if you can, maybe with someone else of similar pace. Hopefully you can build to two or 3 20+ milers.
PS Try to fit in a half-marathon race 4-7 weeks ahead of marathon.