The nearest major international airport is Toulouse-Blagnac which is, on average, a 1 hour 45 minute – 2 hour drive from Najac. There are a good selection of car hire companies at the airport.
Some international flights do come into Rodez-Aveyron airport, just over an hour from Najac, but these aren’t year round. Worth checking online.
The drive from Paris takes approximately 6 hours and, on a good day, it takes 9 hours from the Eurotunnel terminus at Calais-Coquelles to Najac. The major source of delay is usually the transit through and round Paris: it is best to avoid the rush hours and holiday Saturday mornings.
Najac has its own railway station and there are frequent trains from Toulouse-Matabiau station which take approximately 90 minutes along a particularly scenic route. There are three trains each day from Paris (Gare Montparnasse) which connect with trains to Najac, two via Toulouse and one overnight train via Capdenac.