On what surfaces can I safely use my hybrid bike? I will be doing a 50 mile ride in a few weeks' time, most of which is on tow paths and a disused railway line. The surface will therefore be somewhat rough and loose, but not muddy and no major obstacles.

Obviously I would like to use the bike I have, which I use for commuting every day on smooth tarmac bike paths.
It is a hybrid bike (rigid forks, flat handlebars, regular rim brakes), which is obviously not a major problem, but I have replaced the original tires (35mm I think,and a little knobbly) with smooth 28mm tires.

Will my bike be suitable, comfortable and safe (for the bike and for the rider) on off-road tracks?

Edit for more info:
The disused railway is fully decommissioned, and is routinely used by walkers and cyclists (it is now a public bridleway). I haven't seen the surface personally but I gather it is fairly level and smooth but covered in a mixture of compacted soil, gravel and stone chippings and grass in some areas
28mm wide tires are perfectly suitable for riding hard-packed fine gravel trail surfaces. You do have to be more careful when braking and cornering than on tarmac as the top surface is loose. Larger rocks and potholes should be avoided of course. 

As the size of the gravel particles gets larger or the depth of the loose gravel top surface gets deeper the less suitable 28mm tires will become.

I personally will ride 28mm tires on deeper, coarser gravel for short sections, but I would not want to do it for 50 miles. As well as increased likelihood of punctures ad discomfort, narrow tires take more power to push through deeper gravel as then tend to sink in rather than floating over the top.

Note that on flat hard-pack fine gravel you can decrease your tire pressure for better grip and comfort, but on deeper coarse gravel higher pressures are needed to avoid  pinch-flats.  



Update


  [The surface] is fairly level and smooth but covered in a mixture of compacted soil, gravel and stone chippings and grass in some areas.


You'll be better off with your original 35mm tires. With the larger tires you will not have to worry about what you will encounter, and you will be able to run lower pressure for better comfort.