how much on-road use is too much?

I've put nearly 2000kms on my stock trailwings, they have plenty of tread, but i just want something a bit more capable in the dirt. i was looking at the Michelin t63, but am now leaning towards the dunlop d606.
the only problem is, i ride my bike a fair bit on the road. up to 60kms a day. so how much is too much for the d606s? will they handle this?

My 606's have over 5000 km's on them...I want a new set, but there is meat left.
I'm guessing over half those km's are tarmac.
For an all round tire they will be hard to beat.

I usually got 5000 solid miles out of a rear D606, and about 3500 out of the front.