Ultra question

You'll be alright with the sling?

Run lots! 

What ultra are you looking to do?  McNaughton is April 10th. 

I ran it last year.  We'd do long runs starting at 3 hours and building to 7 Jan-Mar (the race was April 11th last year) with a 2 week "taper" (I was training for IMCdA, so I was riding and swimming a lot too, even during the taper).  A couple weekends, I'd run 12-16 miles on Saturday and another 8 on Sunday, otherwise, my long run was about 7 hours (28 miles or so).  I'd usually get in 2 short runs mid-week, so the training schedule was very heavy on the long run (50-70% of my weekly miles were the long run). 

For most people, it isn't practical to try to make your long run 70-80% of the ultra distance (like is typical with a marathon plan) but getting some decent miles a day after your long run or splitting your long run over two days works pretty well. 

I think if you have a few good long runs under your belt then you would be good to go on top of regular daily mileage.  You did an IM-I think you could easily do a 50 miler-it's a different exhaustion but you can push through.   I would try and run some hills bc the terrain will be more difficult than a road race.  Good Luck and have fun!