Ultramarathon Program by George Parrott

Hal Hidgon's "How to Train" contains a chapter on training for ultras. George Parrott was the coach of the Buffalo Chips, an ultramarathon club that produced many ultra champs. He's also a professor of psychology at CSU Sacromento.

The 50-K
"Fifty km is simply a marathon with a warmup."
Pacing is key: run 26 miles with a split time 15 to 20 min slower than the marathon best (35 to 45 sec per mile - 20 to 30 sec per km - slower). Then run the last 5 miles at a pace 60 sec per mile (35 sec per km) slower.

For first timer, choose a point-to-point or large loop course rather than one with multiple laps. This is because everyone experiences periods of mental and physical fatigue. Avoid races made more difficult by terrain, high altitude or chance of extreme weather. Sounds like good advice for any ultra, not only a 50K.

The Sandwich Approach (back-to-back tempo and long runs, sandwiched by rest days):
Mon: rest
Tue: speedwork (warm up, 3x1 mile, 2 - 3 min jog between, cooldown) at faster than 10 k pace (6 miles)
Wed: easy run (8 miles)
Thu: easy run (6 miles)
Fri: rest
Sat: tempo run (warmup 2 miles, marathon pace 3 miles, cooldown) (7 miles)
Sun: long run (walk breaks) 23 miles
Total: 50 miles

Increase mileage in speedwork, easy and long runs (but not tempo run) each week to build up to 54 miles before taper three weeks before ultramarathon. A 10-K race the week before as a final test of conditioning. For the experienced runner, this program adds one or two months to the usual marathon training buildup.

The 50-Mile
Moving from the 50-K to the 50-mile requires considerably more elan, but maybe not that much more talent or training. Parrott's program does not require great increases in mileage, merely tweaking the schedule and spending some extra time on the weekends -- long runs on both Sat and Sun.

The weekday schedule is unchanged. For the weekend, instead of miles, substitute with time:

Sat: 3 hours easy
Sun: 3 to 4 hours easy
Week total: 60 miles

Recovery week every third week with only one long run on the weekend. 

Increase Sat run to 4 - 5 hours and reduce Sun run to 3 to 3.5 hours on weeks 6 and 7 (week total of 75 miles) before tapering. The last 2 hours on Sun should be at increased heart rate level (80%).

42 miles (67 km over 6 - 7 hours) over the weekends is unlikely given my pace, so my actual mileage will be lower.