Bit later than planned but here you go:
The bit on the A222 before you get on the A20 cycle track is a main road, but it's downhill which I always find makes it substantially less stressful. There's a signed bit that gets you into the ped/cycle subway thing; when you're in it I think the signs to look for are South Subway and then St Mary's Cray.
That route takes the main road from Eynsford to Shoreham; you can also turn right onto Castle Rd just south of Shoreham and then hang a left at the signpost. It's much nicer, though does feature a steeper hill.
If you wanted a nicer route back in you could head for Cudham and then Biggin Hill after Star Hill and get onto Layhams, and come in through West Wickham. Though the A21's usually not too bad after about 8pm, and it does have a cycle path (well, shared use pavement, but there are basically no pedestrians at that time until you get to Bromley proper) alongside it.