Cycling, huh?
O.K., brace yourself for too much advice.
First thing is to get a cycle which fits you and also fits your style of cycling. It could be anything from a full blown racer with carbon frame and skinny tyres to a sit up and beg shopper. Do you already have a bike? You may need to adjust things like the saddle to make it comfortable for a long ride. I expect that you will be in the saddle for around 5 hours each day - even 3 hours, a lunch break, then another 2 hours is a long time in the saddle and you have to be really, really comfortable.
Second thing is to build yourself up gently. If you don't do much exercise then setting out on a long 4 day bike ride is likely to end in tears. If your aim is to average 55 miles each day then it would be good to have done at least 40 miles in a day before you start.
Third thing; is there a local cycling club? You may need advice on bike fit, pedalling style and cadence, best sort of clothing to wear and many other things. You can also get good advice from a GOOD local bike shop. The trick is to find a good one.
Fourth thing; can you find a training buddy? Like walking and running, it makes a lot of difference if you have somebody to cycle with. You encourage each other to go out for a ride when the weather isn't nice or you are feeling unmotivated and you tend to go at a slightly faster pace when there are two or more of you cycling.
O.K. - no more counting! Proper cycling shorts (with built in padding) worn next to the skin are a must for me to avoid soreness down below. I used Baby Cream on the padding before a ride and it keeps my bum from ending up chafed. A rear view mirror is a great help to keep an eye on what is behind you. Lycra (yeah, I know about MAMILs) is very suitable for cycling. Light, warm, and not too sweaty. There is a reason why serious cyclists wear it and it isn't so they are mistaken for TdF contenders. Reflective clothing is a very good thing on long rides. Being visible is generally a good thing. Mudguards are a great help. It may well rain at some point and it can be a pain to have a streak of muddy water up your back and your front. Good lights are a must; some cyclists have flashing lights during the day just to make themselves more visible but they are necessary in case of bad weather or being out after dusk.
Plan on an average speed of 11 mph when doing your route planning, unless you have already done your 40 miles at a much higher average speed without even breaking into a sweat.
Be aware that hills can ruin your whole day. If your long distance route has more hills than your training route allow extra time and also expect to be more tired than you expected. There is a reason that touring bikes tend to have mountain bike gearing; at some time you are going to be tired and faced with a long steep hill so having a very low gear and just plugging away can be a lot better than having to get off and walk.
I assume that you will be pre-booking accommodation and not carrying a tent with you.
Finally, on diet.
Since I started on LCHF I find that I can ride up to 50k (about 35 miles) on my morning cup of coffee, butter and double cream. As far as I can remember I couldn't do that when I was running on carbohydrate. So you need to decide if you are going to be fuelled by fat or by carbohydrates and plan your meals to suit. Allegedly fat burning is better for endurance rides. YMMV.
