Well to start you should be doing more than 40km per week by Peak ironman Training, a Normal peak Triathlon week would generally include 5 runs, 1 x Sunday long run (3hrs), 2 x Interval Hill Sessions of around 75-90 Minutes each and 2 small Brick Runs ( 1 hard after a Mid Week race pace brick session 60min bike/30run etc and a 20-30mins easy run after Saturday mornings Long ride), thus you are already on 60-70kms per week (5-6min per km) on "Tired" legs. Thus when you are dropping 2 sports and upping your running week to 90-100kms on "fresh running legs" its not that much of a jump. Also the jump from 30km to 65km can be made easier by getting the qualifier 42km in first and of course you have done a 42km on TIRED legs at the end of Ironman only weeks before. A good 50% of Novice Comrades runners (including me on my 1st in 2012) jump straight from 42km to 90kms without doing a single distance in between. The other 50% have got in either an ultra or a Comrades Long run (all the jhb running clubs have an organised "water pointed" one, RAC is prob the best, so dont do this alone if possible). Thus why i say the Race Day for Comrades is more of a "Beast" as unlike Ironman where you have swum a 3.8km, cycled a 180km, and prob run at least 35kms in training individually. This time you are at your Max trained distance only half way in.
