Townsend, 48, will be joined by former Scotland colleagues Kenny Logan, the 70-times-capped wing, and Stuart Grimes, the 71-times-capped lock, and England and Lions prop Phil Vickery on a trek through the High Atlas Mountains in north Africa.
Ex-Gala and Border Reivers star Townsend said: “I’ve really only climbed the Eildon Hills before.
“We’ll cover 50 kilometres over the three days, and we’ll reach a height of about 2,500 metres above sea level.
“We’ll camp overnight, and we’ll finish near Marrakech in Morocco.
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“We’re doing this to raise vital funds for the My Name’5 Doddie Foundation, the charity Doddie founded as part of his courageous battle with motor neurone disease.
“Since his diagnosis, Doddie has pushed very hard to find a cure for this terrible disease and also to support fellow sufferers.
“His efforts are helping fund vital MND research and are moving us closer to the day that we will be able to finally cure MND. He is a true inspiration.”
To donate, go to https://www.justgiving.com/fundraising/gregor-townsend
Former Melrose and Border Reivers player Weir, 51, of Blainslie, was diagnosed with MND at the end of 2016 and went on to found the My Name’5 Doddie Foundation to help fellow sufferers and fund research into the disease in a bid to find a cure or better treatments.