Running ahhhh. Different mentality.
Think about it almost starting from zero on the bike. You don't want to hurt yourself.
Start at maybe 15min per run every other day. Over a couple weeks add 5min per run. No intensity. Z1/Z2 level running. Maybe even Galloway run/walk.
Then, after a couple months, add some intensity. 4x400's and such. Hill sprints. Just one of those runs per week. Do a 5k test or mile test. THEN get into a real training plan once you have some bases.
I think a lot of the intro running stuff ramps the distance or time too quickly to get those "bucket list" 1/2 or full marathoners to the finish line.
Once you have a base running, keep it. Don't treat it as a once per month "filler" for not riding the bike. Otherwise you'll only be doing low Z2 or Z1 running junk miles. If you keep up with it, you can do a legit "filler" workout to substitute the bike.
It's taken me a few months, but I can now go do a 40ish minute tempo run at lunch anytime a bike that day isn't an option. If it is a bike heavy week, I just do a couple 30min runs that week to warmup before the bike or while kids watch tv before bed to keep it up.
40ish min at tempo on a well established routine is good aerobic work towards the bike. 20min of awkward junk miles once per month is nothing but inviting injury and a waste of time. I'd say do core/yoga instead if that's all it will be.
I'm not a duathlete, but I feel any self respecting cyclist should be able to run a 5k healthily any day. Meaning, not in pain and able to 'get after it'. And their 5k time should be within a minute either side of what they could do a 10mi TT in on the bicycle.
It's taken a couple years, but I'm at about 22min for both right now on an "optimal" day and route. And I'm very pleased with that. I'm not elite in either, still slow for a true runner's 5k time by far. But I feel more balanced athletically.