Back breaking work when you aren't used to it but good news all around.
Set a string line 48" from the garage as I am laying a row of concrete bricks to separate the gravel from the grass, then dug out along the string line. Matock and shovel work to trench out where the corrugated drain piping will go.
Then discovered that there is a line coming out on the SW corner of the garage that is clear to the NW corner of the garage, which means that I can trench out the north side, connect to the NW corner, and connect the SW corner to the line I am laying now. Much easier than I thought, since I was expecting to trench all the way to the edge of the yard by the street.
Off to the local hardware store for connectors where I picked up two Ts, a 90 degree connector, and a downspout connector. Turns out you can connect the gutter downspout right to the drain line. Who knew? (Everyone except me probably but I grew up in one of those "plumbers have to make a living too" homes where I didn't learn how to fix much of anything.)
Hooray. A giant sense of accomplishment plus I stopped early enough that I hopefully won't be in agony tomorrow (being in shape for martial arts, weightlifting, and paddle-boarding does not, as it turns out, mean that one is in shape to dig drainage ditches).
Tomorrow a truck full (Toyota so a small full) of 1-1/4" crushed clean gravel and I'm going to try to get the pipe laid (minds out of the gutter boys), the gravel back filled, and the rest of the blocks laid. Then all I'll have to do is the north side which should be easier since it has only one connection.
PS 1-1/4" crushed clean because I was told that it packs down nicely, whereas drain rock doesn't so is hard to walk on.