It all depends on the terrain you will be covering, if its mainly hard tarmac and the occasional off road section, then well cushioned trainers would be my personal choice.
If its a 36mile mountain walk, then well broken in boots, that you know to be comfortable, and will not give you blisters or discomfort.
Many years ago, i completed one of the toughest challenge walks in South Wales, the now discontinued South Wales Marathon walk, one end of the Brecon Beacons National Park to the other, a distance of around 53miles, over some very challenging terrain.
I could complete the distance in around 14hrs, so depending on the terrain, 15hrs is a very achievable target time for 36miles , as long as you stay fully hydrated.
You can never carry enough water for walks of this length, and remaining hydrated is half the battle of success on challenge walks.
When you get slightly dehydrated, you eventually develop muscle cramps and soreness, and any early sighs of leg discomfort, can lead to extreme fatigue earlier than is necessary, especially when you have trained for your particular event.
36miles is a good distance to challenge ones fitness levels, and correct footwear and staying fully hydrated for as long as possibly are two of the most important criteria for success.
If you can walk 36miles, then there is no reason why you cannot walk 50miles, its mostly mind over matter, as long as your body is in fit shape, both mentally and physically.
Staying hydrated at all times, and eating small amounts at regular intervals, is what you should try and do, especially if you have never walked 36miles before, its still a long way for a first time attempt, and if the weather is warm and humid, then it will be a stern challenge.
Ive passed the 70mile barrier three times in the past, and it was only severe muscle soreness than finished me off, and i recon severe dehydration came into play as well.
When your shattered and trying desperately to stay on course and on time for the annual LDWA hundred, its inevitable you will forget to drink enough fluid, and eat regularly.
The Camelbak hydration packs were not around in the early 1980s, when i was at the peak of my challenge walking career, they would have come in very handy.
If the walk is on hard unforgiving tarmac for most of the way, then go for trainers, if its mostly off road, tried and tested boots, but drink small amounts regularly, and eat small amounts as well.
Good Luck.