Other answers have touched on this but not explicitly. The main safety point I would say is how you ride when you're tired. If you're exhausted and can't keep a straight line up a busy hill, don't think to look over your shoulder in time etc., that's bad and getting dangerous. tiredness can really hit your judgement and reaction times.
Otherwise it depends on your starting level of fitness and facilities in work more than safety. I started from an occasional leisure ride by riding the 9-10 mile each way commute once a week for a couple of weeks, then twice, but not consecutive days. Now I'll happily do 4 days in a week if it fits in, and would try every day but haven't had the chance.
I would say that the bike doesn't make that much difference at the scale of your ride or mine - with the obvious caveat that it fits reasonably well and isn't in a terrible condition or something weird. If it's OK on a cruiser it will be better on a road bike or hybrid (my choice for urban riding given the state of the roads/hills and the need to see over cars), but if you find it horrible on a cruiser, you wouldn't find it a pleasure on an unfamiliar road bike.
Of course, if you're at risk of heart attacks, recovering from an injury or whatever it's a different matter.