A 20 day Camino would require you to walk 38KMs per day. The record as far as I know is 8 days, achieved by an American woman some years ago, averaging 100KMs a day. She ran, finding the conditions and terrain relatively easy, an epic achievement.
I think that a 21 day Camino Francsise is very achievable without too much stress, provided the weather is good, you walk without interuption, are fit, strong, well organised and lucky.
If you love heat then you should have no problems, if not then you are likely to struggle as the heat tends to slow you down through dehydration, exhaustion and limit your walking time to early mornings and evenings and how many pilgrims will head out walking in the evening after a rest break in the middle of the dsy? Walking in the dark is likely to be slower than in day light and more prone to errors.
I walked my fastest
Camino Frances from Saint Jean Pied de Porte to Santiago in 13 days. My first Camino took 15 Days; which is my average for "At Pace" caminos. My first Camino was all over the place in terms of KMs walked per day, from 25KMs to 85KMs at the extremes, as I had no plan, it evolved.
Here is some information on my 13 Day Camino, which I planned as a 14 day Camino. I walked in November 2013, with perfectly cool weather conditions ie no rain and dry ground. I found the walking to be effortless, except on Day 4, when I struggled due to lack of sleep the night before so walked for just 8.5 hours over a 11 hour day covering just 44KMs. My first camino was epic in terms of sense of achievement and my 13 day was both epic and effortless.
By effortless I mean that I was having fun at all times, in the zone, easy going even when my body was tired. I did not over stretch myself except on the last day, when it did not matter so much, but still had endless fun walking through the night under a full moon, surrounded by the howling guard dogs of Galacia.
Day 1 SJPP to Pamplona - 67.00 KMs
Day 2 Pamplona to Los Arcos - 67.00
Day 3 Los Arcos to Najera- 57.60
Day 4 Najera to Belerado - 44.00
Day 5 Belo to Rabe de la Calzada 61.00
Day 6 Rabe to Carrion De Los Condes 53.00
Day 7 Carrion to Sahagun. 60.00
Day 8 Sagagun to Leon. 56.25
Day 9 Leon to Murias De Rachivido 55.00
Day 10 Murias to Cacabelos. 64.00
Day 11 Cacsbelos to Triastela. 60.00
Day 12 Triastela to Airexe. 61.50
Day 13 Airexe to Santiago. 76.00
Total KMs. 782.25KMs
The official distsnce is 789.10KMs (
Brierley)
So I lost 7KMs somewhere !!!!
Average Hours walking per Day. 9.0 HRs
Average Hours (breaks and walking) 12 HRs
Shortest Day. 11.00 HRs (8HRs walking)
Longest Day. 17.25 HRs
Average Moving Speed 6.50 KPH
Net Average Speed. 5.00 KPH
I do not normally walk 6.5 KPH, usually 5.5 KPH, so I must have been much lighter in weight than normal and somewhat fitter.
Average Heart Rate. 105
Elivation Gain. 13,018 M
Elivation Loss. 13,122 M
Average KMs per day - 60.17 KMs
Brieley KMs per day. - 60.69 KMs
Good luck with your Camino, whatever time you take to complete it.