Watson is musical director of a 12-hour Vivaldi concert at the Royal Festival Hall in London on 6 September. He agreed to take the job provided he did not have to stop for meal breaks.
Raymond Gubbay, the promoter, contacted a doctor who has agreed to stand by to attach Watson to a glucose drip whenever his energy level flags.
Three groups of musicians will take part. While one performs, the other two will be rehearsing. Watson will oversee the event and conduct the last section. The musicians will be allowed to rest - but not in a style to which they are accustomed.
Backstage futons are being provided for the Mozart Festival Orchestra during breaks. Mr Gubbay said: 'It is such a heavy schedule, with rehearsals and performances overlapping, that he will be racing around from concert platform to rehearsal and back to the platform with no free time.
'He will probably have three meals by drip during the day but they will also be given to him during rehearsals.
'This was the only way he could oversee the marathon. I had heard about it being done in America and thought we could try it out here. If it is successful we may see a lot more of it.'
In all, 21 of Vivaldi's works will be performed, including The Four Seasons, the piece to be conducted by Watson.
Mr Gubbay said: 'We're concluding with that piece and we're particularly anxious to make sure Ian Watson is fit at the end of the day and has enough sustenance for all four of the Seasons.'