I’m in a kind of similar position. Training for a HM (in May), running 3 days a week. I started running in October as i’d lost interest in the bike (not really sure why, but that’s for another post).
I have a duathlon in September, and I don’t want to be completely unfit on the bike come Spring so I was thinking of trying one of the Sprint Tri base phases first as they’re only 2 bike sessions a week and low TSS (45 min sessions). I want any cycling to compliment the HM training right now (rather than detract). I’m also commuting ~10 miles a day 3-4 days a week.
I’ve never given that much time to train (when racing my local track league i was only training 2 - 3 days a week) and I want to keep that life/training balance. Maybe I’m asking too much of my time and should just focus on the HM and then include some bike June - September before the Duathlon.