You put physialogical stress/strain on your body when lifting escpecially if you are close to max. You are not recovered the day after if your HR continues to be elevated even though you feel you are. This happens to me after leg day and also after a heavy bench day. You need to plan your taining more appropriately to allow for adequate recovery. Most triathletes seems to have a problem doing this as the feel they will lose fitness if they don't train every day. I used to be one of them but I have found that letting myself recover will benefit me more over time.

You didn't mention...are you a runner or multisport? The main different between the two is scheduling in this regard.


Short answer, yes. 85-90% 1RM strength phase training is going to have a significant impact on endurance performance. Depending on your goals that may (or may not) be appropriate at this time. It is generally incompatible with a heavy endurance build phase, and this is especially true for any intensity training. So, if your goal is strength building, I'd back down on the long run...and just make it more of a medium run. Once you've finished with your strength phase, go back to a maintenance phase with 3x8-12reps, 2x per week. Then work to extend the long run. If your goal is building your run-endurance, then I'd switch to strength maintenance, now.

Doing your long run slow isn't a bad thing. And the elevated HR isn't really, either. For most people the long run is just about time-on-feet---and frankly, the slower the better. And HR is just sorta what it is. As long as you know "why" its elevated, and your pace is "slowish", and RPE is ok, its not a big deal.

The bigger concern is with respect to potential for injury---depending on your history. When your muscles are fatigued from a heavy lift day, your running form is likely to suffer similarly to your pace/hr. So, you will be at some elevated risk of injury. This is "doubly" true at the end of a long run---you were in a pre-fatigued state at the start, and your form isn't going to improve at the end of 20k. A wrong step, or a goofy misfire, or a muscle cramp...and, bam your sitting it out for a week.

There's also the issue of stacked fatigue from the back-to-back "hard days" causing excess trailing recovery...leading to slower rate of improvement (or none, or even regression).

So, I would deconflict the two events...by putting a recovery workout or off-day between them. In reality though, answering this question, in total, requires knowing what the rest of your week looks like. You have an opportunity for 3 (maybe 4 if you're multisport) hard days for the week..separated by 36-48 hours. How do you want to spend them?

First of all, thanks for the quick answers! Forgot to mention some background: last full year was focused on 70.3 training (4.15 PB) until april, when I lost all motivation to train sort off. Currently in graduation period + work means less time to train, and wanted to 'focus' on running this fall/winter. With my body composition (194cm/6ft4) I do have to slowly progress, so currently I'm running 3 times per week, wanting to add a 4th run soon. Those are all easy except 1 where I add in some fartleks on hills. Besides that I might add in a Bouldering session, some yoga and/or bike ride(s).

Reading the advice on strength periodization is really helpful, kind off forgot about it to be honest. Currently the gym stuff is not that structured and just off of feel. Will be good to get some structure in, and also just consolidate strength through a maintenance program. Might also switch workouts to have the 4th recovery run after gym day or just complete rest (or bike/yoga)...

Training for longer endurance events I pretty much ditched using heart rate for anything. Way too many variables. I've learned to go by rate of perceived exertion. I also pretty much ditched the power meter as well....

I recently started experimenting with positive split training.....

15 to 20 km are 1:15 h to 1:40 h and is not a real-real long run yet. I think something like that should be doable the day after strength training. Running slowly is another training than strength training so they should not influence their benefit mutually. Clearly you're more tired but I would indeed run as you do with slower speed, maybe even slower as you do now to get to HR 145 again. Do not run by distance, but by time, 1:30 in your case.
I would say that the day after strength training you should not do something high intensity or a hard build training, but a long slow jog should be exactly that which is suitable on such a day (the alternative is doing nothing after strength training, but if you do strength training 2 times a week as you do that would leave only three days for doing something different).