divagate, depart, straggle, wander, stray, sidetrack, divert, deviate, decline, step aside, more...

progress, advance

meander, ramble, wander, drift, deviate, digression, more...

get sidetracked,   rambling on,   go on a tangent,   go off topic,   get off topic,   to go on a tangent,   veer onto a tangent,   to go off topic,   go off task