4.35 is still a good first mara time. Certainly sounds like sub 4 is achievable. I just sneaked under at London and most of mine have been around that.
All looking good if you're already doing 21 mile runs and your body is still enthusiastic about doing more! I'm nowhere near that distance yet and stamina is a real problem for me. I can do a 1.40 half now but tend to run out of gas on the longer runs. Partly my fault in the past by not doing the proper training, liking beer too much and not building it up slowly! When I did train well and ate well I was down to 3.35.
You've got me at it now, bad influence!! Autumn options, Liverpool, Great Langdale, Berlin! Need to go to Marathon runners anonymous !