You'd have wanted to work these out months ago.

However, if you have nothing to go on - Then I have two ways to get at it.

1.a Ride 20 bpm below your zone 5 for heart rate. Since you use heart rate I am assuming you have done this test.
1.b Run th first four miles at 15 bpm below your zone 5 heart rate. It should be obvious but your zone 5 for bike and zone 5 for run are not the same number.

The other way is to flip it around.
1.a. On the bike take in 1.75 calories per pound of body weight per hour. Ride as fast as you can and still have your stomach and guts be happy and doing their thing. If your stomach goes south on you, you are going too fast, back way off, let your stomach recover and get back to a sensible effort.
1.b On the run, 1.5.

Ideally you would have used those pacing plans during race simulation workouts already, but you didn't so you need to pick something.

As you've guessed, it is a rare situation indeed when a person forced to walk in a race isn't having an issue with nutrition, pacing or hydration.

Good luck

I agree this should have been considered months ago but it was a bad run at my "A" race, 2 weeks ago, which has prompted me to use this lesser race as an experimental exercise.

I had a go, last week, at biking to a HR which I had calculated to be roughly in the right zone and it felt surprisingly tough considering I think I need to back off the bike more... Since then I've also come down with some cold symptoms and so was hoping there would be a quick and easy way suggested to give me a HR to attempt to hold for bike.
Based on some previous data, my threshold bike HR is 165 and threshold run HR is 175. Am I therefore looking at 145 and 160 as sensible values?

I feel that proper nutrition is going to be a great starting point too.

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Date: 10/31/2017

[quote jac2689

I agree this should have been considered months ago but it was a bad run at my "A" race, 2 weeks ago, which has prompted me to use this lesser race as an experimental exercise.

I had a go, last week, at biking to a HR which I had calculated to be roughly in the right zone and it felt surprisingly tough considering I think I need to back off the bike more... Since then I've also come down with some cold symptoms and so was hoping there would be a quick and easy way suggested to give me a HR to attempt to hold for bike.
Based on some previous data, my threshold bike HR is 165 and threshold run HR is 175. Am I therefore looking at 145 and 160 as sensible values?

I feel that proper nutrition is going to be a great starting point too.[/quote]
Seems like you already have an upper limit on heart rate that you know is too high, and you also have a lower level of calorie intake you know is too low. So the problem is bounded.

You can double check heart rate zones by looking at the highest heart rate you have ever held for 30 minutes. If you're a trainingpeaks user that is easy to get.

The reason you went downhill on the run is very likely a lack of calories, so turn the thinking around. No matter how you set your pace, whether heart rate, power, thighs burning, rpe - it all comes back to calories. If the proper rate of calories are being absorbed you are OK. And if they aren't being absorbed, no matter what the watch or power meter is telling you - you are not going to be OK.

Bike: 20 beats below threshold HR until mile 40 and then make an assessment of your current state.

Run: Start 10 beats above Bike HR

With a 5K of 19:35 you should be running well under 1:40 in a HIM. If you don't, either the courses are really hard or you over-biked for your current fitness.