I'd take Bloor/Danforth all the way to Kingston and then over to Brimley. Kingston road/any road south of Danforth inevitably leads you up a steep climb unless you come up Coxwell (which I think is the nicer of all the uphill major streets to Danforth level). That being said, it's nice to ride near the lake on a day like this, and there's not much of this weather left.

Cosburn is a little north, but has the bike lane - Mortimer is fast, but can have rush hour traffic on a fair bit. May as well take Danforth proper, IMO.