You definitely can substitute a smartphone for a GPS.
Whether you should or shouldn't and how are matters for debate.
1. Keeping the screen on all the time depletes your battery quite fast. You'll be very lucky if your phone's batteries last a full day. Screen off, even with GPS tracking should last the (short) day. As some have suggested, a battery pack will come in handy.
2. GPS or phone, a decent mount is a good idea.
3. In the rain, phones are not responding well to touches. They'll react to raindrops and if they aren't waterproof, they might stop working.
4. If you go with a phone, you probably want to install a navigation app and the necessary maps. I like locus pro. Many people prefer Osmand.
5. For your next trip, you are probably better carrying both.