It really depends on which stretch you are doing - from personal experience the SWCP seems to have more cafes, beach bars etc than the Pembrokeshire Coastal path where often there was no chance of refreshment over the whole days walk especially on the northern part of the path.
In the more touristy areas where there are lots of beaches and resorts there should be lots of chances to restock on calories - other stretches such as from St Ives to Pendeen have less available en route - if you join SWCP association they publish a very useful guide book advising of what is available on each stretch.
As I recall from book 500 mile walks written by Mark Wallington about his walk along SWCP with a canine friend he lived on muesli for the whole walk…...
Also be careful not to underestimate the terrain and how it will affect the distance travelled - especially with carrying camping gear etc.
A rule of thumb for coastal walking is that it is always further than you think, harder than you think and usually will take longer than you think.