Hey Michael!
Great question- especially relevant for all of us in the Northern Hemisphere! I enjoyed digging into some research on this. I’ll do my best to describe what I learned, to best answer your question.
Firstly, a high Volume Training Plan puts a lot of strain on your body already so you will want to be careful when adding in Heat Acclimition strategies. That being said, you are in the fortunate position where you have lots of time, meaning you can do this straegicially and sensibly for the best outcome.
In 2018, Heathcote and colleagues conducted a systematic review on passive heat acclimation strategies for endurance athletes. They found 16 relevant articles. I’m going to do my best to summarise the takeaways that are applicable to your situation.
When to get started?
The earlier the better. You will reap benefits of heat training pretty quickly- maybe as little as a couple of days. But ideally, you’ll want to do heat acclimation strategies for at least 6-7 consecutive days immediately prior to your event. While heat acclimation can occur quickly, it is also lost quickly.
What?
I suggest you start with passive heating strategies. This will protect the quality of your training and allow your body to gradually get used to the added stress.
As you become more heat acclimated and you’re sure you can handle more stress on top of your High Volume Training Plan, you may wish to introduce active strategies, as you described.
How?
The goal is to elevate your core temperature to around 38.5 degrees celsius. This paper reviewed 3 strategies for doing so: saunas, hot water immersion (baths), and heat chambers.
Hot baths are likely the most practical strategy- they take the least amount of time to elevate core temperature (30 - 60 minutes is recommended) and most people have access to a bath tub!
When?
The authors of this study suggest that athletes should commence hot water immersion immediately after exercise since your temperature is likely already elevated.
In summary:
Based on the information gathered from this review, for successful passive heat acclimation, the heat stimulus should ideally have a minimum duration of 30min per session, employed on consecutive days, when possible.
I hope this helps, Micheal! Let me know if you have any questions on this.
Resources
Here are some resources that you might find interesting and helpful:
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How to Get Faster with Heat Training
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The Science of Getting Faster Podcast- Heat Training and Performance with Dr Chirs Minson: https://youtu.be/b8edDmyhHes?si=BZ8B2FGhKbArO_W1
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Heathcote SL, Hassmén P, Zhou S and Stevens CJ (2018) Passive Heating: Reviewing Practical Heat Acclimation Strategies for Endurance Athletes. Front. Physiol. 9:1851. doi: 10.3389/fphys.2018.01851