Well this will be my first ever running race I have ever entered... so I thought it may be good to go all out and just enter into a 100km instead of a 10km...

It will be an excellent challenge for me, and this first time I am just aiming to beat cut-off and finish. The longest distance I have ever run is about 15km, although I have been competing in 24hr rogaines and 8-12hour adventure races.
I just have a few questions about building mileage. I am no runner, I come from a competitive swimming background so I do not have years of experience under my belt. I have been running training for about a year now, and in the past sit somewhere around 20km a week, but in the last few months I have been trying to raise the bar in preparation for the NF100.
I am abiding by the 10% increase rule, but every few weeks something happens. Either I get no time to train, or I get an injury. After recovering I try to return to the mileage only to be utterly defeated by it. As such, I have only really reached a mark of about 40km per week and I know that this will not be enough KMs in the legs for me to have a shot in May.
I was thinking about an alternative to a 10% increase per week, just so I can get some much needed mileage. If I stuck with a set mileage a week, and started maybe with a 40/60 run/walk method, then increased the running ratio instead of the running mileage, would that work? From rogaining I have plenty of experience walking at a fast pace for many many hours.
For eg:
Week 1: 50km (40:60 Run:Walk)
Week 2: 55km (45:55 Run:Walk)
Week 3: 60km (50:50 Run:Walk)
Week 4: 65km (55:45 Run:Walk)
etc etc until I have reached a good mileage, and a run:walk ratio that I plan on using on the race.
All my training is done offroad on similar gradient trails to the NF100.