Let's see if I understand your question.  You are doing a tempo or interval workout plus an MP run every week and want to notch it up a bit by adding a second run some days.  Wish suggested adding hill sessions, which really would benefit you, but you are unsure of how to work them in.  You know, you don't have to do everything every week, which would be overkill.  You are already switching off the tempo and interval runs.  Why not do the same with hills and MP runs?

Personally, I think you will gain more from hill sessions than MP runs and should do a few of them for every MP run....like one MP run every 4 weeks and hills the other weeks.  MP runs simply train you to become accustomed to and “comfortable” with running at MP and, if they are long enough, might offer some threshold benefit in the late miles….that’s it. OTOH, hills train you to be a stronger runner....period!

I don't think you get max training benefit from "hard" sessions by training to be “comfortable” at any pace, such as MP. I think “hard” workouts should be very challenging, which means getting out of the comfort zone.  If you train each of your running systems (AT, aerobic capacity, anaerobic capacity, running economy and strength) to a peak, your optimum MP will be "comfortable" to about 20 miles.  Hill repeats contribute to the development of all of the running systems (particurlarly strength, aerobic capacity and anaerobic capacity)....MP runs don't.  I think the primary benefit of MP runs is to become familiar with the pace so that you don't overrun the first half on race day.  They do not really prepare you to go the distance at that pace.  In other words, I think you can get more benefit from other forms of hard running than you can from MP runs.  Even without hill sessions, I would prefer both a threshold and interval workout most weeks instead of one of a MP run and one of the others.

Also, since you live in a flat part of the country, Wish's treadmill suggestion is a very good one.  In fact, Jack Daniels says that he prefers hills to be run on a 'mill because incline and pace can be precisely controlled.  If you don't have access to a treadmill, look for overpasses or parking garages.  But, find a way to run hill repeats if you want to gain that additional 10 minutes that Wish mentioned.

I also agree with Wish that a second run a couple of days a week probably won't do much for you....and it just might cost you valuable rest time and detract from the quality of your hard sessions.  If you are looking for a way to add mileage to your program and still preserve rest periods, I think it's better to increase the length of the easy runs a mile or two.

An even better way to add both intensity and mileage is to do the hill sessions on a relatively short mileage day and replace one of your easy runs with a medium long run of 2/3-3/4 the length of the previous weekend's long run, which could include some miles at MP.

For what it's worth, here's a schedule I used for several years in the '80s with good results.

Mon - Hard, quality run

Tues - 6 miles easy

Wed - Medium long run of 2/3-3/4 the length of previous weekends long run at easy pace.

Thur - Hard, quality run

Fri - Off

Sat - Long run

Sun - 6 miles easy

Hard, quality runs were tempo runs/cruise intervals, VO2max intervals, or hill repeats.  I ran only 2-3 dedicated MP runs during a 14-16 week training cycle, all late in the training cycle, although I did mix in MP running during some long runs, medium long runs and easy runs.  I also included weight workouts 2-3 times per week....usually Tuesday before the easy run, Thursday before the hard run and/or Saturday after the long run.  My schedule started at about 40 miles per week and peaked at 60-65 as the long and medium long runs increased.

On the 3-5 weeks out of 14-16 that I raced, which I tried to schedule races on weeks that I cut back on the long run, I usually ran my cut-back long run on Sat, raced Sunday (usually 8-10k), and took an off day Mon, then picked up my program as described above.  Essentially, the race on Sunday replaced the Monday hard session, the "back-to-back" hard days of a moderately long run Sat and race Sun added strengthening challenge, and the two easy days (Mon-Tues) permitted good recovery.  I ran these races as hard as I could, considering that I did not taper for them. These "training races" were usually strong and not far off my PR times.

I can't do that much anymore, but only because I'm no longer a “youngster” of 45-50 years old.  I never reached your level of 3-hours.  I was a plodder with best times of a 40:58 10k and a couple of 3:22 marathons.  But, you are probably younger and a better runner than I was. 

Hope there's a thought or two in all of this rambling that might be helpful in your program.