To endure and enjoy the TA, your body needs to be well prepared, maintained along the way and occasionally fixed up due to accumulated wear 'n' tear or as a result of a perfectly executed over-the-handlebars lawn dart.
It's a huge challenge to eat a sufficient quantity and quality of food over the duration of your TA. Critically, you'll gobble up huge amounts of fat, protein and salt - especially if it's hot. Electrolyte supplements are essential. An end of day protein drink to aid recovery may also be useful. Multi vitamins are worth considering to help prevent mineral depletion.
Test it all before the start so you know what's normal for you and what agrees or doesn't with your constitution and cravings. Pay attention to your pee, noting the quantity and colour - clear and copious is a nice target to strive for. Read more about nutrition here.
Take great care of your bum. The very first step is to sort out your bike fit, choice of saddle and short to minimise pressure and chaffing points. Then develop your daily program. Most will use a chamois cream to prevent saddle sores, some in excess - a rare few will get away without it. Some like thinner, natural oil-based products while others prefer thicker creams like Sudocreme (conveniently available at Four Square).
An important part of the formula is keeping your bum dry. Some riders sprinkle their shorts with talcum powder instead of using a chamois cream. Others swear by the combination of a traditional Brooks saddle with a chamois-less liner like the Ground Effect Flatliners.
Most importantly extensively test well before the actual event, find what works for you - and stick with it on the TA.
At the end of each day, get your shorts off and air your bum (where decency allows). Lather up with soap and warm water, or wet wipes if water is scarce (wet wipes are not so great environmentally - use sparingly and dispose in the trash - not buried, flushed or long dropped). This is especially important if you're a chamois cream user. Letting soft skin dry is important, and the sun has wonderful anti-microbial properties. A healthy cloud of anti bacterial talcum powder on the nethers can also help.
If you're going minimalist with just a single pair of shorts, then wash them daily when possible. It's okay to put them on damp the following morning - they'll dry quickly as you ride. Most however will take a spare pair so you can 'wear one pair - wash the other' in rotation.
Numb buns, neck, elbow, wrist and/or hands are real and potentially show-stopping issues. As outlined above, make sure your bike fit is sorted to distribute weight between your hands and bum. For 'further reading' Jeff Lyall has an excellent blog detailing his experience and remedies.
Do everything practical to get the weight off your back and onto your bike - using bikepacking baggage rather than a backpack. Avoiding just a few kilos added weight on your bum can make a big difference to its enduring health.
Finish each day with 5 - 10 minutes of stretching. The pigeon pose, gentle back bends, hip flexor and hamstring stretches are useful basics to have in your quiver. In addition, whenever you break during the day consider treating your body to a micro stretch with one or more of these exercises.
Swollen achilles often cause grief. Poor conditioning (not spending enough time riding a loaded bike or pushing it uphill), insufficiently stiff shoes or incorrect cleat position may be the culprits. Tips on basic strapping technique from a friendly physio can help mitigate any pain in the field.
Get current with your first aid skills. An appropriate instructional course could help your buddy, if not yourself. Pack appropriate consumables:
The Aide kit from Ground Effect is a useful starting point to build your personalised first aid kit.
And finally - lather up with sunscreen - paying special attention to the back of your calves, top of your thighs, face, 'v' and back of neck. And a proper SPF lip balm in abundance to prevent blistered Mick Jaggers.