I too completed HIMUK with a 1:28 half marathon at the end.
I come from a running background, and until 18 months ago couldn't swim a single stroke front crawl!
Did HIMUK last year, and Full Ironman in France this June.
The secret to running in a Tri is to practice running off the bike at least once a week.
For the Ironman I was cycling for 6 to 6.5 hours (95 to 100 miles), getting off the bike and forcing myself to run for at least 45 mins - sometimes up to 90 mins.
Doing a half Ironman was like dropping down to sprint distance after the full, but I lost the plot a little on the bike, due to being cold.
A session that works for me is a 40 mile bike at an average speed of 18mph, (ish)followed by a 10 mile run at 6.75 to 7 min / mile pace - in the last couple of minutes of the bike ride "spin" your legs at a cadence of over 100 rpm to get rid of some of the lactic in the quads, then you'll find the running easier.
Well done !
Paul.