A low-lying placenta at 21 weeks is quite common, and in most women it moves further away from the cervix as the uterus grows over the coming months. Being 3.5 cm from the internal os right now doesn't rule out this normal shift happening well before you deliver.
Next Steps
Please check with your obstetrician before this trip since they know your full scan and history. If they're comfortable with it, break the 7 hour drive every hour or so to walk briefly, stay hydrated, and avoid heavy lifting or jerky movements.
Health Tips
Stop the journey and get checked right away if you notice any vaginal bleeding, spotting, or cramping.