Threshold felt tougher than expected – race this weekend – input?

You could've felt off for a variety of reasons - sleep, stress, etc. Unless it's been a trend for a couple (or more likely, several) weeks there probably isn't any reason to worry about it. People have off days all the time.

Yes the session still had impact - you only cut it 5 min short. Cutting one session 5 min short would have little to no impact on general fitness.

With a race (presumably one you care about) in less than a week, it was definitely the smart thing to do to cut the workout slightly short if you were feeling off.

My next session goal is 4-3-2-1-1min (1min Float) plus some Hills (3 days out)

Any thoughts on pacing goals?

20 minutes continuous at threshold is so easy to get wrong. If you had done 4-5x5' with a 60" jog, you would have felt much more under control and had those extra five or ten minutes of training volume.

Threshold it's a zone between your LT1 and LT2, if 3:34/km was feeling harder than supposed you could run it at 3:40/km and still have the benefits of the workout, people tend to run threshold too fast.

You still have 6 days ahead so its perfectly fine, you can do 8/10x200(at 3k or mile pace) 2 days before just to sharpen the legs, the other days just run EASY

Whats your most recent race times? Looks like you’re basing your threshold pace off goal pace and not date pace. So most likely you need to slow down!

Having not raced in two years, you are overestimating your current aerobic fitness and hence, had more difficulty with the threshold than anticipated. Handling 45s repeats at 5k pace, even on 30s rest, is not much of an indicator of longer distance fitness. I would guess that that is not your current 5k pace. The race will tell you where you are at and you can keep building over the summer and get there. As said above, use an estimate of date pace, not goal pace, for your thresholds.

Working towards sub-16/sub-34 so what are your current PBs? 16 & 33 align more than 16 & 34. Could indicate that strength is something to work on. Threshold for a 16-flat 5k runner would be around 5:30/mile. 3:34/k might be LT1. Seems like continuous running @ 3:34/k should be pretty tough for you. Do you have workouts where you cruise at that pace? Looks like you might've just gone off a little too hard for fitness. You're pretty close to 10k pace, which might not always feel good for 15min on a random workout day. It's very possible that you're trying to train for where you want your threshold to be. Train for current fitness. Use shorter reps @ T to get more comfortable at faster paces.

You are either 1)overtraining and not rested enough or 2)the pace you think your threshold is is actually too fast.

You’re likely getting a bit of pre-race nerves due to it being your first race in two years. My advice is to not read into it too much, this race will be a rust buster. Also this is just one step on the journey to sub 16/34, so just focus on going out and giving the race your best, but don’t read into the results too much either way. Just focus on building your base and getting in shape to hit your goals. Best of luck!

Bro, you're basically me with the times and workouts I run, I'm 21 and running for fun in college with similar goals. I notice that tough weeks at school/work can make a huge impact on my performance, even at practice, as others have said sleep/stress. Also depending on relationship status and what you're doing in your "extracurricular time" can significantly impact you. I'd also say maybe check your shoes, check the weather, and check your nutrition/hydration.

Other than all that, if I'm having a crappy threshold session, instead of just calling it, I usually take 5 and get a full recovery, change up my music, I'll switch to a dry shirt that isn't sweaty, grab some water, and fully mentally reset. For me at least, that helps significantly to get my focus back and feel more comfortable. With threshold, preserving the quality of the session seems more important, and I for one, believe that if you need the extra rest in order to keep working at the level you're looking to work at during that session, you should take it.

Hope this helps, good luck!