I am also trying to dial in some straight-up duathlon training. I started by replacing my swims with bikes since running is my relative strength, but I realized yesterday I need to get more time on my feet for HR reasons.

I imagine training principles are the same. I personally like the 80/20 Triathlon book and principle. I also like the idea of keeping your easy days easy and your hard days hard. For example, I have heard several coaches say on podcasts they schedule hard bike and running sessions the same day (with proper rest) rather than back to back days. But if you aren't doubling up, I think alternating hard and easy days makes the most sense. I also do core and what strength I can do at home after hard work outs. My easy days are very easy and I feel very rested the following day.

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80/20 Endurance Ambassador

Run most mornings. Ride most afternoon/evenings.

Day off when my legs are sore.

I’m doing an 8 week bike focus. 3 steady runs per week (around 8:15 per mile) for about 20 miles a week. 4 rides totaling about 6-8 hours depending on the week. 1 mid week high end ride with power above FTP, 2 endurance rides, 1 with tempo work

It looks exactly like the 14th week of the 80/20 level 3 Olympic plan.

Here's a super unscientific answer for you...but given how unpredictable things are right now you may dig it. I mostly run, still bike casually for the most part, don't swim at all anymore. I try to get in a progression cutdown run and long run (1:45-2:00) weekly, and barring injury keep my miles at 50-70. Ideally the cutdown is midweek (Tues), and the long run is Saturday or Sunday. I like to do back to back longer days (say 120min on Sat/90 on Sun or vice versa) with my running. Thursday is whatever I feel like, could be long-ish and easy in the hills or shorter but at least an hour, I listen to what I'm feeling. Outside of that, I try to get in 2x45-50min easy runs on some combination of M/W/F, typically on W/F.

I usually bike M/W/F for 1-2h. One of those is a Sufferfest workout (Wednesday), one will have tempo or sprints and ideally be 2h (Friday), and Monday is usually 90ish minutes with some activation sprints out of the saddle to stay mobile. I do strength stuff after my Tues/Thurs runs.

Here's that laid out:

M - 90min ride w/short sprints (easy day!)
T - 10-12mi run w/cutdown (70-80min), 30mins of high intensity strength stuff
W - 50-75min ride w/hard intervals (Sufferfest) + 50min easy run
TH - Usually 10-14mi (no longer than 1h45) easy running in the hills, 30mins strength stuff (keep this to 10mi if you feel dead inside)
F - 120min ride w/some sort of intervals (keep it tame before the weekend), 45-50min run is always intended but often scrapped
S - 80-120min run
S - 80-120min run (I try to get in a 2h run every weekend, some weekends I end up with back to back 75-90min runs, sometimes one of the two weekend runs becomes a trainer ride if the weather sucks - life happens)

Some version of that has been what I've defaulted to ever since I decided I didn't really care about triathlon anymore and just wanted to bike and run for fun. It can be accomplished in one training session per day, or you can break stuff up. If you want to double, add in easy biking for 60min wherever you want. My bike won't improve off of this anymore, pretty much just maintenance, I sit right at 4-4.3w/Kg on this, and my running continues to improve (looking for 1:17-18 in a HM later this year if I can find the right race, and maybe a marathon or ultra).

It's the most fun I've had training, I took my favorite workouts (long runs, short intense bike sessions) and got rid of what I don't enjoy (track workouts, swimming).

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"Don't you have to go be stupid somewhere else?"..."Not until 4!"

I’m doing TrainerRoad sweet spot base high volume which is 5 hardish workouts a week and one easy. I add on another easy one to ride 7 days a week. I’ve cut my run down to 30 mpw (from 40-50) for maintenance, BarryP style.

Week looks like this:
M: 6 mile “easy” run, 60 min endurance ride
T: 90 min sweet spot workout, 3 mile recovery run
W: 6 mile “easy” run, 60 min sweet spot workout
Th: 90 min sweet spot workout, 3 mile recovery run
F: 9 mile long run, 60 min endurance ride
S: 2 hr sweet spot workout
S: 2 hr sweet spot workout, 3 mile recovery run

Long time duathlete (Since 1984).
My structure is pretty much the same year round.
Morning run (usually around 5 to 6 miles) 45:00.
Evening ride (usually an hour to an hour and a half)
I do "hard days" on both run and bike on Tuesday and Thursday. AM is running repeats of either 400's or 1000's. PM are bike intervals (usually 4x6:00 hard 2:00 easy or a tempo ride with 30:00 at tempo effort.)
Saturday I do a tempo run with 25:00 at tempo pace.
I only compete in sprint distance races so my longest run is usually around an hour and my longest bike is 1.5 hours.