Hello,
Yes Corsica is a great island for walking tours.
The GR20 is full on with a loaded pack on day one coming to the high ground from Calinzana...around 1,600 mtrs. plus the distance....an early start when the air is cooler is to be sought after. Additionally you have the two litres of water to include. We also came in from Calinzana a year later to travel the first three stages of the GR20. On this occasion it rained from half way until we reached the camping at Piobbu which gave us a cooler day in fact more like a home day out. We had two objectives for this trip. To reach Monti Cintu and then get down to Bastia and to reach the start of the MM Nord at Moriani. Later, when I came in from Conca in the South, the first day was more relaxing and also a more gradual easing into walking with the big pack.
MM Nord is very rural walking in the early stages....near empty villages but with fine wee trails connecting. Corte is a splendid town to stay for a couple of days. The Tavignanu mule track up to refuge A Sega is a good day out and a choice of ways to get to Col di Vergio followed by two more days out to reach the sea at Cargese. These days are shared with the Mare e Monti Nord.
We had another journey beginning in Calizana when we connected the M e M Nord and both the Mare e Monti Sud, with a bus into Ajaccio to reach the start, and that led us onto the Mare a Mare Sud at Burgo to a finish in Porto Vecchio.
The Mare a Mare Central was reached by the bus from Bastia for a start. Pleasant in the Central area where you cross the GR20 at the Bocca Laparo.
Of course there is a lighter way to travel in Corse if you elect to stay in the refuges that have to booked in advance or to hire one of the erected tents found at each refuge. For us the carried tent is more flexible for the days where we will end up at for a camp and also at the end of a walk we usually have a couple of days in a coastal camping site so no need for hotel expenses etc.