The Pilgrim's Reception Office is specific about this. It requires at least 100 continuous km on foot on one route, excludes other forms of travel except for pilgrims with disabilities, and expects each day's stamps to show that the pilgrim has resumed the Way in the same place where they last stopped. From Sarria the margin is thin, about 15 km (9 miles) over the minimum, so a skipped section puts the certificate in doubt unless you return to walk it. The cathedral chapter can consider special and exceptional cases, but asks pilgrims to contact the office in advance.
In practice, American Pilgrims on the Camino notes that buses and trains are frequent and inexpensive on the popular routes, and that where there is no service a taxi can usually be arranged; your host can help. If your bag has gone ahead with a transfer service it will be waiting at the booked address, so riding there and returning to your stopping point the next morning is the usual solution. Correos lets you change the delivery address for its bag service until 8:00 pm the evening before, which helps if you decide to shorten the days ahead.
Good to know before you go: Spain's Guardia Civil tells pilgrims to measure their strength and to give up the day's walk if they feel they cannot continue. Stopping is the sensible act. Note the name of the village where you stop and get a stamp there. For anything worse than tiredness, such as a fall, chest pain or heat exhaustion, call 112. If you have doubts before you start, plan shorter stages from the beginning: Galicia's official stage guides list the intermediate villages where a day can end.