Rose Ave to Narrabeen lakes

You'd go Rose, Veterans, Esplanade, Lakeside then along the Narrabeen Lakes path up to the bridge

the bike path around the lake, near pipe clay point, is definitely CX friendly, though not paved.

There are a few very short sandy patches and some slightly rooty sections. If you have 35 mm slick tyres on the urban bike you'll probably survive without too great a risk of flats, though without a lot of grip. Bonus being you don't need to ride up Rose/Veterans, and you get to see some wildlife.

There is a paved cycleway (shared) all the way from Dee Why Beach to James Wheeler Pde (at the bottom of Rose Av), that avoids riding along Pittwater Rd and/or having to go up & over Collaroy Plateau. However, just yesterday I noticed that there is some temporary fencing around the soccer fields at Cromer Park that has closed a section of the cycleway - you need to use Fisher Rd to avoid this part.

From James Wheeler Pde, it continues around the southern shore of Narrabeen Lagoon as an unpaved track, to Jamieson Reserve. It became quite boggy & muddy during all the rain last winter, but the Council have recently laid down some gravel & dirt, so now its relatively smooth. There are a few sections that are a bit rocky with some small loose rubble, but I've seen all sorts of bikes riding along here & never noticed anyone with a flat.

You should be ok, just take a spare tube incase.