EEPROM has been guaranteed to work with 100000 write cycles. If you save rarely enough, it is not problem. With 5 s priod you will reach that less than week.
If you have battery on Teensy, you can use VBAT registers.
struct tNvRamData {
uint32_t EngineRunTime; // Seconds
};
#define NVRAM_BASE 0x4003E000
tNvRamData *pNvRamData=(tNvRamData *)(NVRAM_BASE);
...
pNvRamData->EngineRunTime+=...;
...
tNvRamData has been defined as struct, so you can easily add there more data.
There are totally 32 VBAT registers, but I read somewhere that 0x4003E01C has been used by Teensy code, so there is 28 byte available. This means max. 7 uint32_t.
When you save to SD card, other than interrupts will be stopped.