Build does take a lot out of you… Many examples in the forum.
As its the final effort, it tends to be one of the highest exertion levels. You have said your HR was higher can be affected by many things temperature etc. Is your rest HR higher too? One nap and a better sleep may help a bit.
The main thing it is only one training session. I would do a 60-75% recovery ride and start the recovery week early. No point in starting the training session and having to stop half way through.
By doing 7 weeks of build you will have done sufficient stress to get the training affects.
Good luck when the swimming can start again.