The Flame in the Flood

As a point of reference, the river in the campaign is 45 miles long, so try shooting for having the motor by the time you hit mile 20 or so.

I believe the wording for this achievement is bad because I unlocked this after only using the motor about a dozen times. It must count the number of miles you travel after building the raft motor upgrade, rather than have it activated (holding ) for a cumulative 20 miles. I suggest buying the raft motor as soon as possible so you can get this in one playthorough!

Although it seems that previously this achievement would unlock from simply having the motor built, it appears that this has been fixed and now the achievement will only unlock after the motor has been running for 20 miles. In other words, distance travelled is only counted towards this achievement when you have the motor upgrade and are holding down .

With this in mind, it's still a good idea to try and get the motor as early as possible and have around 4-5 jars of gas so you can fill up the motor frequently and keep it running for as long as possible. The motor can be upgraded at a marina with 3 raft schematics, 3 raft hardware and 3 raft frames. Gas can be filled (using empty jars) at marinas and later on, gas stations.

This achievement tracks over several playthroughs, so you don't need to worry about getting the full 20 miles in one go, it took me around 3-4 playthroughs of the campaign to get this achievement, although I wasn't specifically working towards it so it would be entirely possible to do it in less. This achievement can also be gotten through endless mode or a mixture of the both.