Endurance Training, Is It Enough?

In order to balance being a good husband and father, I ride my mtb. only twice a week.  I get one weekday ride and one weekend ride.  The key has been making the most out of both rides.  I try to make the weekday ride 20-30 miles max with a mix of 6 hr pace and XC pace.  There are always faster riders to try and keep up with, and something about night riding makes you want to go fast anyway, so that is never a problem.
On the weekend I try to get a long 'endurance' type ride in.  40-50 miles that will hopefully increase to 6 to 8 hours of riding.  These rides are usually done at somewhat sustainable pacing.  At Umstead, a 21 mile loop, hitting all the hills can be done twice at a 11.5-12.5 average.  Throw in some single-track and the average drops.  My favorite trail off Old Reedy Creek Rd. is ~a 12 mile loop and a good endurance pace would be ~8.5mph.  Unfortunately, the poor weather, Umstead is all that I have ridden (on the weekends) for the past two or three weeks.  Hopefully this week will lead to some sweet single-track. :)
So, hopefully riding my mtb. twice a week will be enough to train for PMBAR and the Burn24.  As far as Warrior Creek is concerned, I will treat that like a 6 hour training ride and just try to enjoy myself.

Oh, I should mention that I do get to commute to work a few days a week on my SS road bike (gear ratio of 48:17).  It does give me a decent work out, and my average commute is ~17 miles (6 to work and 11 home) sometimes more, sometimes less.