Sustrans probably would be happy with that, unlike other cycle campaigners, hell, Sustrans probably wouldn't care if it didn't have a tarmac surface. There's plenty of their so-called National Cycle Network has their signs pointing down half metre wide muddy tracks that you need to lift your bike over a barrier to access.
Other cycle campaigners would point out that 3.5m is the minimum standard in various government publications for a new build off-road cycleway/mixed use path and that ideally if busier it should be 4m or more, with no barrier that block access to trikes, tandems and trailers and that it should have a tarmac surface for all weather use by all cycles.