Club every time. You will be astonished at the progress you will make. The Bridge Club at Hart has nurtured many new runners into the sport - right up to the completion of marathons for some. Have a look at that section on the website - they are wonderfully supportive of the new runner, and offer a very warm welcome.
Sorry - HUGELY presumptious of me. You mentioned that you were training for half marathons two years ago. Irrespective of whether you are a beginner, a seasoned vet or anything in between Hart R R has something for you.
I started cycling in September from a running background - 14 years of road, trail and xc running with a preference for longer distances - I took it up less because of recurring injury, although there were increasingly more of those, but simply because I wanted to. I just bought a bike and got on with it, initially with a couple of mates who had done a similar thing years before, and I also started riding in a group on Saturdays with a local tri club who have been fantastic sharing their knowledge with me.
I started as a complete numpty but decided that if i was going to do it at all, I'd give it everything I had - s**t or bust - I am 58 now and had not been on a bike since I left school 40 years ago, but I decided that I might as well start as I intended to go on with clipless pedals. All the gear, and no idea )
I simply can not tell you just how much enjoyment I get from it. Owing to the dark evenings I am presently limited to weekend riding but we get out for at least one 55 - 60 mile ride on Saturday or Sunday with a run and a shorter ride ( 25 - 30 miles ) on the other day if at all possible, weather permitting.
I loved running and I still run in the dark mid week evenings, and hopefully at least once at the weekend. However I can already see myself sacrificing most of those evening runs for a bike ride once the nights get light enough.
I can not swim for s**t so the triathlon is highly unlikely to see me, and whilst my cycling ambitions are, at present anyway, targeted towards sportives ( aiming for The Hampshire Hilly Hundred in May ) I may well try the odd duathlon to maintain some sense of running focus and, I remind myself, I love running too.
The point of all this witter is that the only advice that I, a total newcomer, can offer is " Go for it " - go for it with both hands and if you do I really hope that you will get the same enjoyment which I have got. Be warned though. It's hugely compulsive and addictive. I bought my first bike in September. My second one arrives tomorrow.