I admire triathletes, but I've found that for me it's hard to conciliate running and cycling. Either I have a running focus, or I have a cycling focus, but I find it hard to do one properly without compromising the other, for any distance over 10K as far as running is concerned.

If you're preparing for a half marathon, you'll have to build up to long runs of least a 2-hour time, not counting two runs a week of lesser duration. So three runs a week is pretty much necessary.

If you're ready to make that commitment and try something new, then by all means do it!