The tarmac towpath continues around the north of the town centre of Agen, just north of the railway station, and then along the north bank of the canal through mixed commercial and residential areas for perhaps 5-6km before returning to a more rural setting. The run to Moissac however has a more built-up feel than the route all the way from the western end as far as Agen does. There is evidence of a busy road, small industrial estates, and most notably the railway line (Bordeaux to Toulouse) which runs directly alongside the canal much of the way now to Moissac. At times the cycle path is on the other side of the canal, at others it runs between canal and railway as it crosses and re-crosses the canal from time to time on small bridges.
A note on the railway line: the Bordeaux-Toulouse line is never very far away from the Garonne Canal for the entire length of the canal to Toulouse. It passes through La Réole and Marmande for example, and might be 5km or so away at times. It provides some comfort that, in the event of an emergency or a total breakdown in your bike, you would not have very far to go to find a station at which either express or local stopping rail services can be connected to get directly to a city or even return home if necessary. For much of the way from Agen to Toulouse you could access the next station simply by sticking to the canal towpath/cycle route until you see it right beside the canal. If there is no-one in the ticket office, and no automatic ticket dispenser, no worries - the ticket inspectors will almost certainly find you on the train and willingly accept that you couldn't get a ticket before boarding. They just sell you one on the spot without fuss. Fares are competitive compared to the UK.
As you approach Moissac, route signs for the Véloroute des 2 Mers direct you south of the town centre towards the embankment of the Tarn river, which is fine if you are passing straight through as it brings you back to the south bank of the canal where the towpath/cycle path continues, but you can also continue straight though a short section with "Quais", or canal-side streets, either side. The main town centre is just to the north of this section of canal.
Moissac is considerably smaller than Agen, but it has bars, restaurants and shops to belie its size. It has a very posh hotel on the banks of the Tarn (Moulin de Moissac) but other more modest establishments in the town centre. The main historic attraction is L'Abbaye St-Pierre, in the north-centre of the town close to the main square.
Geotagged snaps can be found here.