I know there are a bunch of threads on this, several of which I have now read. I signed up for Strava again today… What I see according to their algorithm is a decrease in fitness (not that I am sure why). Sigh. So I wonder whether that is because with the low volume plan I actually have less time on the bike than before I started TR. Now that I’ve searched the forum, I see that lots of people supplement the LV plan with other rides. Does anyone have a recommendation for how to do that? On days that I am not dead after the workout and have time, the most practical thing would be to add some time afterwards. Do I want to go into Train Now and pick something? Will that mess up my training plan? I have, for example Venado +1 for this afternoon. Should I try to add a zone 2 workout after that? My goal, in theory, is to be able to ride a 100-mile event in August. There are shorter routes that day, so it’s no big deal if I don’t manage that, but I do want to get as fast as I can before that.
This is the single most common and universally accepted way to supplement any training plan, whether TR or not.
If you can accomplish your key workouts, you are really giving yourself a nice boost by adding Zone 2. Either add at the end of the TR workout or add another day if you feel up to it.
Also, ignore that Strava fitness thing.
Stay consistent and stay healthy. 
I’m pretty sure they just use a HR based TSS equivalent. If you’re spending less time training the number goes down
I agree, but also don’t. I’d say add as much endurance as your fatigue permits. You can add as much Z2 as you like, but if that means you won’t be able to complete the harder LV workouts, cut it back a bit.
First off ignore Strava’s Algorithm.
You don’t need to overthink it just get on your bike and ride. If you’ve not got time, etc for outdoors riding just select ‘Train Now’ and put in the time you do have and it’ll come up with a workout recommendation based on your recent fitness level. Your training plan will adapt round that and do the thinking for you.
There are some really good podcasts out there, featuring top coaches/scientists and the advice for everyday athletes is pretty much the same - volume and consistency and progression.
As a general principle that means if you can train 8 hours a week, but 3 days of intervals is so exhausting you can only do 5 hours a week, you have a good chance of improving performance by cutting back on the number of intervals and riding 7 or 8 hours a week.
Am I understanding you correctly that if I have a few more hours it is better to use them. So if I have my three TR workouts I should add a couple or three hours and not worry if my performance in the TR workouts is not as good as it might have been? I am just trying to get into shape to ride as far and as fast as I can, but with the idea that that will not really be fast. So grinding every last bit out of the intense workouts might not be necessary and the extra mileage would provide a better benefit. Maybe?
Do the low volume plans 3 scheduled workouts, then add a couple “endurance” days. Have a easy ride on Wednesday and then a long ride or whatever length you can manage on Sunday. Make sure you can nail the 3 TrainerRoad workouts and watch what happens 
My example “low volume plan”week if you notice I even extend time on Tuesday and Thursday. This works for me because my body is used to it. If all you do for a long ride is 2 hours that’s better than no hours.
… or you could consider the polarised base and polarised build (mid volume) which would give you two days of intensity and two days of endurance per week.
Yeah…I was going to say dump the Thurs SS and do a long Z2 ride. Flex the Wed ride as either easy recovery spinning or rest day. Basically…Polarized.
Some good posts above. I’ve killed my self doing 8 hours/week, lots of really hard and intense efforts. And lately been doing mostly easy 8 hours/week. Fitness wise I ended up in about the same place but I’m older and need more recovery. Both better than when doing 5-6 hours/week. The volume principle worked for me. Worth considering and again some good suggestions on how to do it in TR posted above