i rode to paris with a postie bag and although it's relatively comfortable, i'm seconding the suggestions for you to get something more touring oriented... as for your arse, bring a pair of padded cycling shorts!
@silasmellor started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
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i rode to paris with a postie bag and although it's relatively comfortable, i'm seconding the suggestions for you to get something more touring oriented... as for your arse, bring a pair of padded cycling shorts!