Just started a training plan and have a question.
My plan includes;
1 day of 6-8 short Z5 intervals
1 day of 2 long Z4 intervals
1 day of 2hrs Z3/tempo
1 day of 4 hrs Z2 with 10 mins Z4 every hour
The remaining 3 days are 1/2-1hr Z1 recovery.
I'm an avid rider since 20yrs ago, have trained before and have a good base.
My question is, I commute about 45mins each way 5 days per week. If I ride slowly, always in Z1, would it interfere with my training plan?

Personally I think there's too much Z3 and Z4 in there, but that's your business. I'd replace one of the Z3 tempo hours with a 45 minute Z2 commute, and then come back at tempo - take a slightly longer route if you want a full hour at that intensity. In other words, incorporate some of your commuting into your training plan. Extend that principle into your recovery rides. On three days a week, your commutes can be the recovery rides you describe. That's four days commuting sorted out, without your having increased the training load.

I used to do this myself when I was commuting about 65 minutes each way. The commutes provided about 10 hours of base training per week, mostly z2 stuff. Supplementing that with a couple of interval sessions and one long ride at the weekend and I was good to go.

No.

Tangentially you want to look into polarized training with much more Z2, no Z3, and probably more Z5 time. That should do at least as well increasing your VO2max, be better for your lactate threshold, and be much better for your sustainable speed on longer rides.

If I understand your question, you want to know if you can bike commute 45 minutes each way in zone 1, AND do your training program described in your post (as opposed to training DURING your commute).

You can. I have a similiar commute in terms in length, and bike commuted while training for Ironman. The Zone 1 commuting will be good recovery, but please let us know how you're fairing after the first month (I have no comment with regards to the actual structure of your training plan).

You are talking about 7.5 hours of commuting plus 10.5 to 12 hours of training. That seems like a lot to me. One of the beauties of bike commuting is you can reduce a little bit of your training time (particularly zone 1 and zone 2 recovery rides).

Thanks all. Maybe I wasn't clear enough, my question was if adding commuting time to training time would do any harm, as cvskates wrote. I could use 3 commuting days as recovery days, but I'd like to keep hard days apart.
I'm not totally sure about this plan. It was recommended to me but I could change it. My previous training wasn't as structured as this one. Do you think it'd be better to skip th Z3 workout and do more Z2 instead? As I said, I have a good base, used to ride about 15000kms a year and though I ride less now, still is 10000kms a year at least, and can ride a slow century no problems.

What is your commute like? Is it all urban start and stop or do you have any long uninterrupted stretches? If it's the latter, there's no reason you can't incorporate your interval training into the commute itself.

Half of it is road, the other half city streets. The road part could be used for short intervals, but I'd feel uncomfortable in street clothes and on a IGH bike.

It's not practical to ride your road bike?

No. I ride 2-2.5 hours of Z1 for my on-the-bike rest days (it makes me more relaxed) and it works fine. Obviously you need the discipline to go slow enough to recover, even if children fly past on tricycles. Treat it as a game if you need to - I was thrilled to average 95 bpm. It also counts as volume for fatigue purposes so don't increase your weekly total riding time too quickly.

Yes. Keep it below your aerobic threshold and you'll raise that so you can ride a fast century no problems, or above your anaerobic threshold so you can ride fast for shorter distances.