eh? it’s just a Sportive. there’s no need for a training plan just gradually increase the distance on your longer rides as and when you have time.
Despite Sportives not being races many people want to ride them as fast as possible. Having a training plan will help to get the most out of the riding that you do in the time available. Just riding more is not a good way of getting faster.
So would the best thing to be to ride a much as possible till Skinny Tweed then after that start a proper 12 week training plan for Tour o the Borders?
Sounds reasonable. As mentioned earlier the usual suspects in road mags have 12 week sportive training plans which usually start off fairly easily then adds more distance during the weekend rides each week for 12 weeks while doing intervals mid week. Something like that will build your endurance base while also training your body to recover quickly from hard efforts/hills etc.