With the extra training benefit you could conceivably have a HM PB around 1:32 - 1:33 now ... add 7mins to that time and times by 2 gives you 3:20 fairly comfortably. So perhaps head out on a 3:20 schedule and see what happens?
When I ran 3:16 ..... I couldn't believe how easy 3:09 pace felt - until about half way... and then the wheels started to fall off a bit - and then a bit more..... but that was on fairly average / minimal training.... my HM PB at the time was 1:29 ..... its now 1:24 so in theory the sub 3 hours is doable - hmm
I think you need to revise up your target pace from 3:30 currently to more like 3:10 if you are finding 30k at 5min ks v easy.... personally i wouldn't go above 32-34k there doesn't seem to be much benefit in that it seems v how much it will damage.... better to sling in a threshold run to see how fast you really are. (total run around the 20km mark) ... I do one of these a week as a medium long run midweek.... and then a 5min / km or 8min Mile longer run if I remember at the weekend hehe
ie do a few k to warm up then - run at threshold pace which is basically as fast as you can run "out of breath" so only able to have broken conversation - that should be just over your HM pace.... and not far off your true potential marathon pace.
I ran my first marathon in 4:52 (Dressed as a giant dog) ... and the second in 3:16 .... but the 4:52 was alot more painful over the last few miles.... best to get it out the way quicker I reckon
Def not going longer than 20 .... I'm thinking that a fatigued 17-18 miler would have the same benefit anyway providing you do a hard session the day before...... Pfitzinger vs Daniels approach.... my old coach who was an ex-international runner said he reckoned you never need to train more than half your race distance.... thats an interesting one hehe
Doing Valencia in 5 weeks time .... and have been doing 30 miles a week, with minimal proper long runs - quite a few 11-13 milers.... does anyone swear by the not running more than 16 mile rule for a sub 3 marathon? .... I think it "COULD" be the way forward ..... it seems that anyone trying to train off a "out the book" training program has a very high rate of injury.... prob over 50% get some sort of niggle which just doesn't seem right.
Absolutely will be great psychologically if you are wolving the 20milers down like there is no tomorrow? However, consider the situation where someone starts to really breakdown mentally at 15 miles and drags themselves through the last 5 perhaps after a heavy training week and injures themselves. Obviously for the race itself you may do a 2 or 3 week taper reducing mileage each week to line up fresh....
My first marathon I did one 18 miler 2 weeks before.... along with about 30 miles a week steady and ran 3:16 ... admittedly the last 6 were tough.
You sound like an EM bob (endurance Monster) ie you find the longer stuff easier than the shorter speedier stuff.... hence why you relish the longer long run.... if it works for you stick with it. Its just the mantra that seems to be hammered throughout the marathon running community that your longest 5 runs should equal 100 miles ... frankly I think weighing up the evidence this is not right for the majority .... Having said that I haven't done a proper long run yet so best get out there and do a 20 miler ....my aim for Valencia is 2:59:07.... may need to hire a moped