Fucken hate it when that happens... The most unimportant things are suddenly the most important - but how to motivate yourself, hmmm...
TBH I'd probably sub it out to a kid/friend/relative/tealady with a scanner (or at least a PC and lend them my scanner) and pay them what that part of the job was worth to me.
If this is totally not an option motivate yourself with the fact that if you don't do then you won't be able to finish the project and you'll look like a cunt to your client. Word-of-mouth is the best source of business and bad word-of-mouth is the best way to avoid it, (that or having blaze in your name).
You can also do other stuff while it's scanning of course so set one going and do something in between scans - or process each image after it's scanned, crop it and optimize it etc. I'd avoid wanking as a mid-scan activity as you don't want to return fireplace brochures with the pages stuck together.