Tomas – If you are struggling to run slower than 7:40 pace in training then you may be faster than you think, or not working hard enough on hard days. I believe hard days should be hard enough to leave you needing to run very slow the following day to recover.
How about adding a mid week run of about 13 – 15 miles into your schedule? This could contain HMP and MP paced intervals and would help fatigue you before your LSR.
I’d also recommend trying a half marathon or if you can’t find one soon a 10k race to see how fast you can really race. Once you get a race result you can use an online calculator to see if you are on course for a sub 3:15 or faster.
As others have said most of your long runs should be about ‘time on your feet’ and not distance. I’m targeting 6:45 pace in London but ran my last LSR at 8:00 pace, which meant I was on my feet a bit longer than I hope to be on race day.
I’m happy with my 4 x 1 mile 10k paced intervals this morning in an 8 mile run. I averaged about 6:05 which is dead on 10k pace and this was only a few days after a 24 miler.