Working on that journey like you. Went from about 300 ( 21st7 ) down to 20 - with some training and not enough dieting or family support.
attempted a 70.3 and managed to make it to the first lap of the run before the maths said I would miss the cut off. SO it is doable.
Hindsight : ( things I wish I had done )
1. Get the family on board early.
2. 70.3 is a solid step. but add a sprint and an olympic and a half marathon as mid term stepping stones - boost confidence, confirms progress, ensures family faith you wont die
3. weight first and always. Get the diet cutting back the lard. this needs to work, exercise or not. its unpleasant carrying all that extra over 55miles on the bike and then facing a run with your legs buggered.
4. lots of veterans (iron folk - not just old folk ) here. probe for advice and support often.
5. find a club. get some coaching.
6. skip spending. you don't need anything carbon, fast, expensive, power metery, etc until you finish the long course. ( repeats are always faster apparently )
7. rince and repeat. track your progress openly. not every snack is ruination. snacking every day is.
8. what everyone else said.
good luck. enter your half ( outlaw seems flat but their are lots of events and you only have to ask ) and let everyone know ( here ) how your getting on.
ran 2.5miles yesterday - HRT on the ceiling, kept moving until a 'break for nature' - was a little faster again on average ( and a lot faster without a 'no1' rest pause ). I ll have it up on starve as soon as I find the ant+ dongle.
one worry is that keeping the HRT down around 80% means walking. running pushes up to 184. walking sucks - its not making my legs stronger unless im on 10% incline on the treadmill.
( now my max should be 172 by the 220-age nonsense, and its likely more 188 by evidence but should I be running for 10+ minutes beyond threshold ? )
Or am I confusing myself with the HRT/Threshold thing and just need to keep running harder , for longer then further ?