Letting your body get depleated is never a good idea. Not only does it increase recovery time but is decreases the effectiveness of the workout. In terms of the format of your long run I would say it depends.
- How many marathons have you ran
- How fast are you
- Are you 100% focused on the marathon or more of a triathete doing a marathon
- How many years have you been running
- and a lot more, a good local coach can help you a lot …
I can tell you that the marathoner I coach rarely does “slow” long runs during the final marathon preperation. I think of these as strength training for the muscles and tendon’s and they have minimal to no influance on aerobic fitness in terms of the marathon (note I didn’t say Ironman). Also it is very hard for anyone to run longer then 2:30 in one shot so no matter what pace you run I would cap the total duration at 2:30. For one marathoner I work with if she is running 135 to 140 HR that will be about 7:00 to 7:15 per mile. For another local lady I work with that will be 8:30-8:45 per mile, but in both cases they don’t run over 2:30, the faster athlete just runs farther in that time.
To answer your question you are best served to work up to workouts like this:
30:00 easy, 30:00 @ 90% of MP, 30:00 @ MP, 30:00 all out
also
20:00 easy, 40:00 @ 90%, 35:00 @ 94%, 30:00 @ 98% of MP
you get the idea? You always want to run your longer workouts in a progresssive manner. Sometimes you really crank at the end and sometimes you run strong all the way. The marathoner above ran 20 miles in 2:03 after a 1.5 mile jog before Boston this year and ended up running 2:38 in that race. She spent a significant amount of time doing long progressive runs and only did two slow 2:30 duration runs.
I hope this helps. It is very hard to give advice over a board. The people I coach I work with directly and let there bodies reaction to training load determine the next workouts. I can tell you that this seems to work:
6 - 8 week cycle focusing on all qualites of running:
Build from 1:15 to 1:45 or 1:50 duration long runs over hills
Tempo runs from 3 to 10 miles in length ( a great one is 7 miles start 1:00 slower then goal MP and take off 10 sec per mile so the last mile is at goal MP)
Strides
Low key 5k - 10k races
Fartlek workouts hard
fun running circuit workouts
etc…
3 weeks
focusing on marathon pace 2 x per week or recovery running with high volume
1 week at 50%-60% volume all easy with strides and maybe finish with a race (10k to 15k) at the end of the week. don’t expect great results at the race
3 weeks at about 85 to 90% of the avg volume for the previous 3 week cycle before the rest
2 workouts per week have strides during or after the runs
1 workout per week focus on threshold strength 2 - 3 x {5,4,3,2,1} with 1:00 rests etc … make it up 30 - 45:00 worth of work and short rest
1 workout per week long progression, let yourself feel good running long and fast, don’t be afraid to take very easy days before and after
2 weeks out at about 70% max volume with strides, light fartlek mid week and a medium duration (9 - 12 miles) run with 4 to 6 miles at race pace on the weekend
final week at about 40% volume not including the race. include a few k’s at race pace now and then to remind the legs of the pace without being tired.
Again a coach working with you is the best way to do this right. It is not possible for me to put together a general marathon training program for people that could be anything from 4:00 to 2:30 marathon runners and likely not runners at all but triathletes 
Hope this helps you,
Ed Alyanak