I'm riding a regular short loop, short enough to push hard all the way for about 45 minutes
I have a speedo and I assess my performance on this loop based on time taken and mainly average speed, but sometimes I feel like I put more effort in but go slower
how else can I measure my performance? do I need to measure power output?

Measure it in a rating of fun 😀 – i.e. if it takes the same amount of time but you enjoy it more then I'd say you're performing better

Grin factor!

Garmin and speed/HR monitor across the course.

Thing is you could be burning out too quickly.

I have taken a 20 loop slower throughout and killed my times as I wasn't burnt out using up my glycogen/atp reserves.

avg speed?

but sometimes I feel like I put more effort in but go slower

That's because we have days when we perform well and days when we don't perform so well. Conditions also play a large factor. One of the local loops which I ride can be as quick as 58mins if I gun it all the way round and it's dry, it usually takes about an hour or maybe an hour and five if I ride fairly quickly but in winter it can take an hour and twenty or sometimes even longer when it's really muddy. I was surprised yesterday because I was feeling a bit tired and decided to make a point of riding easy and concentrating on smoothness, my average speed ended up as 11.1mph instead of about 12.5 or 13.9 if I hammer it…. the ride time was one hour and twelve mins, only five or six minutes slower than it takes if I put loads more effort in to get 1.5mph faster ! I really thought at the pace I was riding it would've taken much longer as I was hardly using any effort.

Kev

exactly, conditions can affect speed, so I guess I need to measure my output – which conditions and outside factors can't affect
am I right in thinking I need to measure my heart rate or maybe power output? the former being easier/ cheaper?

do I need to measure power output?

Yes that's the only way to measure it properly and also its best to do it under controlled conditions as ground and weather conditions will affect your performance outside. See if you can a beg or borrow either a turbo or indoor bike with power measurement and push hard for 45 mins or do a couple of hard 20 min intervals with a 5 min rest in between. Your power output will be at or near your functional threshold power (FTP) which is the power output just under your aerobic threshold. FTP is what you should work on increasing if you want to improve your performance in endurance events like enduros or sportives.

so how do I know what my aerobic threshold is? I guess it's related to heart rate – so do I need to work out my max heart rate and all that gubbins?

Perceived exertion and time taken.

needs to be more scientific, it's the only way I'll enjoy it – it's road riding afterall 🙂

What Goan said, exactly.

I've got the wife pregnant again so there is nowt wrong with my performance! ;0)

Aerobic threshold is crudely related to heart rate. Forget aerobic threshold or heart rates. Fundamentally you need to increase the power you can output for an hour or so at a time. The best way to measure that is to pedal as hard as you can for 45min or an hour or 2 x 20 min and measure your power output. Then work to increase that using interval training. Heart rates, perceived effort etc are crude and indirect measurements of performance. Best way is by power output.