60K looms

I am registered to run a 60K on March 8th...my first ultra. I don't care how fast I run it; I just want to finish. I have been running consistently for 8 years probably averaging 30 mpw during that time and have ran 6 marathons including a trail marathon in December. Training was going great until I injured my back the end of December and was forced to ride a stationary bike for 3 weeks with no running. I resumed training last week with a 10 mile long run and I ran 15 miles today. I am now painfree and all is going well. My question is: with only 3 weeks of training remaining with a one week taper, how long should I make my long runs? I ran really slow today...2.5 hours for 15 miles. I don't want to re injure myself and totally miss the run but I don't want to go in totally unprepared. Like I said I don't care about my time. I only want to finish this first one Any advice?